Last year, when the director Mike Nichols wanted to stage a fabulous launch party for his film Closer, he knew exactly who would get it right. He asked Wolfgang Puck to do the honours at his Beverly Hills restaurant Spago. Puck did him proud, installing a three-level chocolate fountain in the middle of the room so Jude Law and Natalie Portman had somewhere to dip their marshmallows. The year before, when he turned 70, Michael Caine decided he too wanted to throw a big party. In the curious way of Los Angeles, where the famous hang out with each other regardless of how little they might appear to have in common, Caine teamed up with the musician Quincy Jones who was also turning 70. They too decided Puck was the man. They too booked out Spago. Stevie Wonder sang happy birthday; Harrison Ford, Calista Flockhart, Sharon Stone, Billy Crystal and Joan Collins joined in.

Since 1998 Elton John has held his post-Oscars party there, an event guaranteeing celebrity gridlock. It was the same when über-agent Swifty Lazar hired the joint at Oscar time, when Puck's restaurant was still on Sunset Strip. It was the place where old and new Hollywood met. One year, at Swifty's party, Jimmy Stewart stood on a chair just to get a better view of a breaking pop star called Madonna in a white rhinestone encrusted gown, and her date for the night, Michael Jackson.

I am thinking all about the unimpeachable glamour of rhinestones and Jimmy Stewart and Stevie Wonder singing happy birthday, while standing outside a restaurant belonging to the man who created the circus in which all this happened. It's a branch of Wolfgang Puck Express in the less than glamorous Los Angeles International Airport. I'm suddenly struck by how good his name is. Wolfgang Puck? It sounds like it has been invented by a sharp marketing department. 'Wolfgang' has exactly the right European heft to reassure an American public they are in safe culinary hands. It shouts old country. 'Puck', as in A Midsummer Night's Dream, is playful, skittish and fun. It's easily pronounceable. Wolfgang Puck Express sounds right too, and looks good across the front of this food concession. On one side is a bar, propped up by a few slack-bellied guys slugging beer, eating pizza and watching baseball on the TV. On the other is a curving open kitchen with a wood-fired oven.

It strikes me that I might be able to get a last taste of that glamour, that celebrity stardust, here, on the cheap, before I fly home. I order a Wolfgang Puck chicken Caesar salad and watch as it is made. Reassuringly the leaves are dressed correctly, by being turned with the slick of creamy sauce in a big stainless steel bowl. But the shards of white chicken come from a small plastic bag, as do the curiously regular croutons, and the long-ago grated flakes of cheese. When my salad arrives it eats as it looks. It's dry, flavourless and dull. I know I shouldn't be surprised. I ordered food in an international airport and it stank ...

But it carries the name of Wolfgang Puck, and we know he wasn't invented by marketing men. He is, instead, the greatest celebrity wrangler American catering has ever seen. He's an Austrian immigrant who has lived the American dream. He's one of the men credited with pioneering Californian- Italian food. He's the guy who invented designer pizzas. He writes bestselling books and flogs everything from soups through to self-heating lattes in supermarkets across the land. He has his own shows on cable TV, and has interests in dozens of restaurants from high- end to mid-market to a 50-strong chain of Expresses. In short Wolfgang Puck is America's most successful celebrity chef. And he now happens to be the man whose name is on the worst Caesar salad I have ever eaten.

It is two nights earlier and I am at Spago in Beverly Hills, the restaurant which, in its original Hollywood incarnation, made Puck's name over 20 years ago. Puck himself is nowhere to be seen tonight but I am being looked after. I have, for example, a table in the prime real-estate of the open courtyard. This should give me a perfect vantage point from which to do a bit of celebrity-spotting, a respectable pastime in the world's movie capital. Sadly the opportunities are sparse. Indeed, the whole experience confounds my expectations. I had presumed I would see some serious stars at Spago and eat some mediocre food. Instead I eat some fantastic food, and spot only Sidney Poitier, which is no achievement. According to the cuttings Poitier practically lives at Spago.

Still it means I can concentrate on the food, which deserves it. No designer pizzas for me. That's lunchtime food, I'm told. Instead I get the tasting menu, which is butch and solid and weirdly Austrian. This also wasn't something I was expecting. There's duck with cherries. There is white asparagus. There's veal with morels and a blood-sausage sauce, and lamb with a pommes purée which is so rich in cheese that it is elastic as it is spooned from its copper pan.

Most strikingly, for a Californian restaurant, there is foie gras, lots of it. Only last year fellow Austrian, governor Arnold Schwarzenegger signed into a law banning the sale of foie gras and the force-feeding of geese to produce it. The ban comes into force in 2012. In Los Angeles, a liberal city which has rarely met a cause it didn't like, animal rights issues could be expected to be popular. If so, Puck doesn't care: there is foie gras sautéed with apples. There is foie gras grilled with black cherries and in a preparation flavoured with Chinese 10 spice (why have five when you can double it?). There is a terrine with rhubarb marmalade and a cute seasoned version that is meant to resemble pastrami and does. Kinda. At the end there are pastries and strudels of the sort that would be the toast of Vienna.

I return to the restaurant the next morning to meet Puck. His publicity pictures show a perky looking man - puckish perhaps - with a cheery grin of white teeth and lightly raised eyebrows like cedillas. His books have names like Live, Love, Eat! and Pizza, Pasta and More! and it's clear the image he wants to project is one of informality and fun. But he is into his fifties now and starting to fill out his whites. When he leads me through the restaurant he appears to limp as if more than three decades on his feet have started to take their toll on his hips. He looks a little bored and grumpy when I engage in small talk about the Austrian flavours I encountered during my dinner the previous night, and only brightens when we finally get onto the subject that all self-invented Americans love most: the story of his success.

It is, to be fair, an intriguing one with a relentless upwards curve. Puck, who is now 56, grew up in Austria where his mother was a professional cook, but he trained in a series of French Michelin-starred restaurants. In 1973 he moved to the US, arriving in Los Angeles two years later as chef at Ma Maison, a well-known Hollywood restaurant. 'My roots were always French,' he says, in an accent which still has a light Germanic burr. 'So that was what I cooked first of all. But I was thinking our food culture should reflect the culture around us.' He was struck by the Asiatic influences in LA and early on replaced the canned tuna in a salad Niçoise with seared fresh tuna. 'People only ate the outside. They would say we can't eat raw tuna.' This is a recurring theme with Puck. He comes up with an innovation. The punters reject it at first before falling in love with it and recognising his genius. 'I believe in educating the customers,' he says. 'And myself, too.'

In 1982 he and his then wife Barbara Lazaroff opened Spago - slang for spaghetti - on Sunset Strip, determined to do something more informal. She did the design and worked the media. He cooked. First up, the pizza. 'When I did pizzas the editor at Food and Wine magazine said Wolf is getting crazy.' But Puck says he knew what he was doing. It had struck him that geographically and in climate southern California was most like Provence or Tuscany. 'So we made pizzas with duck sausage and goats cheese and sun-dried tomatoes.' But the one he became most famous for was an accident. 'We ran out of toast for the smoked salmon. I said put it on a pizza base.' A little sour cream underneath. A little caviar on top. It was a hit.

What really made his name, however, wasn't so much the food that people ate; unlike San Francisco and the Napa Valley, Los Angeles has never been a big food town. What matters here are the people who are doing the eating. Within two weeks of opening on Sunset Strip a single table was booked by the director Billy Wilder, Jack Lemmon, Kirk Douglas and, of course, Sidney Poitier. 'And then W magazine said we are the new hot spot.' Puck says, with a casual shrug. 'And we still are.' He leans towards me as if delivering a great confidence. 'If you are in a city like LA and you have the film business and you get known for things like catering the Oscars party it helps.' Indeed it does. Normally, when citing the names of film stars who have eaten in a particular restaurant, it is worth checking the cuttings. Not with Spago. Name the star. They've eaten there. If they haven't they'll be grateful you mistakenly thought they had.

In 1983 he and Lazaroff opened Chinois, in Santa Monica, which was credited with initiating the boom in Pacific rim fusion cooking. 'We were doing tuna sashimi in 1983. It was extraordinary,' Puck says proudly, as if discussing someone else. Together Puck and Lazaroff went to work building an empire. There was Postrio in San Francisco and Granita in Malibu. There was an outside catering company, which began running the Governor's Oscars' Ball. And then he became consultant chef to one of the city's top hotels, the Bel-Air.

'At that point when he was still cooking at Spago he'd always have burn marks on his hands because he'd been working the line,' says Patrick Willis, who was then food and beverage manager for Rosewood Hotels which then owned the Bel-Air. 'I enjoyed working with him because he wasn't pretentious or arrogant. He was a working chef who knew how to add a twist.' Bruno Lopes, the current chef at the Bel-Air, who has worked in LA for many years, agrees. 'He's extremely smart and a great businessman. And he knows exactly how to get the people around him to do the things he wants them to do.' Puck published books, starred in his own TV series and landed a cookery slot on Good Morning America, one of the most watched shows in the country.

In 1992 he spotted a business opportunity when he initiated the boom in Las Vegas restaurants by opening a second branch of Spago at Caesars Palace. Puck's presence in Vegas would eventually attract the likes of Thomas Keller, Alain Ducasse, Charlie Trotter and Guy Savoy to set up ventures in the world's gaming capital. Puck alone has five restaurants in the city. This would have been enough for most people, but at the same time he was developing a frozen food business. 'Johnny Carson [the late talk show host] would take home 10 or 12 of my pizzas at a time,' Puck says. 'I asked him why. He told me he put them in the freezer and then he can get his chef to cook them. When I knew this I started half cooking them for him.' He says that, prepared properly, his frozen pizzas are the best products of their type on the market.

Many people around the restaurant business in LA now quietly credit his ex-wife Lazaroff with shaping the Wolfgang Puck brand, and driving the $300 million empire it underpins. After 19 years of marriage and two sons they split in 2002. Puck has a new partner, fashion designer Gelila Assefa, who gave birth to their son Oliver in July of this year, though Lazaroff remains a part of the business. But clearly, what makes it all work so well is the chef's own enthusiasm for whatever it is he happens to be flogging at that moment. Recently, for example, he launched a range of self-heating lattes. 'Everybody drives here,' he says. 'Everybody wants to have coffee. It really will be very convenient.' I ask where I can purchase one. 'Oh, it all went to the Midwest,' he says, airily. 'It really is a great product.'

In 2001 he went into partnership with ConAgra, a massive supermarket food producer, which now manufactures his frozen pizzas. 'I believe if we want to be a market leader we can't take on Campbell's,' he says. 'So we are moving everything organic. A lot of people are into organic. I'm focusing the direction of our products.' At this point he stops sounding like a chef, and starts sounding like the CEO of a major combine. Gordon Ramsay's recently-announced expansions into Tokyo and New York may make him look like a live wire, but compared to Puck, he's a rank amateur in the chef brand management business.

Perhaps the most controversial of Puck's ventures are the Expresses. 'When we set out what we were going to do I looked at a company like Armani. He got famous for his haute couture, but you can also get Armani jeans and T-shirts, and they trust it because it's Armani.' There are now over 50 of them, which obviously means he can't be in them all, all of the time. 'No and I should not,' he says. 'We need to set up a system which can be executed. You need to have the best people and pay them well.'

The problem, according to his critics, is that instead of being good value, Wolfgang Puck Express has become just plain cheap: the restaurants, with their Chinois chicken salads and four cheese pizzas and 'Live, Love, Eat' slogans, have been accused of hard sell. In one recent critique the Houston Press of Texas mocked the laminated cards titled 'How best to dine at Wolfgang Puck' which, the paper said, came down to little more than 'order a lot of stuff'. A signed chef's jacket 'commemorating the one and only time he ever set foot in the establishment', is in a glass case outside the men's toilet, and on the walls above the urinals, are adverts for his cookbooks. Videos of Puck's TV shows play continuously. All of this, the Houston Press concluded, has nothing to do with the food Puck cooks in his top-end restaurants or the celebrities who eat it. 'The myth created by advertising and marketing has overtaken the reality.'

'People criticise us here and there and that comes with success,' Puck says, before acknowledging that mistakes have been made. 'It's very difficult. A year ago I fired everybody running the Express group. I didn't like the direction it was going. I didn't trust it any more.' Too many graduates of Harvard Business School, he says. Too much shaving off margins. '

With so much going on, I can't help wondering if he finds the time to cook, but he insists that he does. 'I love the kitchen. It's where I am happiest. Hopefully in 10 years time I will have a restaurant with just five or 10 tables, open only five nights a week and I can cook. That's my retirement plan.' Don't the notoriously picky eaters of Los Angeles make it tough to be a chef here? 'Sure, we have the anti-veal lobby. We have the anti-foie gras lobby. But most of the people who do that don't know how the ducks are raised or where they come from. Yes they are force fed but they like it.' And then, 'I'm not going to have people tell me what to do. I'm not going to have that.'

He takes me on a tour of his kitchen with its staff of 40 and its wood-fired pizza ovens and insists that I try a couple before I leave. 'Certain things have a bad name, like pizza,' he says. 'Use the best ingredients though and it can be the best.' I am seated and he sends out a smoked salmon pizza. The secret he says, is that the base should not be too hot when the salmon goes on. Again, I am pleasantly surprised. I had expected a gimmick, but the combination works. It is rich and luxurious and moreish. But it is trumped by the second one that arrives, stacked with morels from Oregon, and shards of confit bacon, asparagus, leeks and fresh parsley. I write in my notebook, 'I have a deep and profound sadness that I cannot eat it all.'

And if my experience of the food of Wolfgang Puck had ended there, this would probably have been one long love letter to the man who is arguably America's most successful chef. That morel and bacon pizza really is a thing of beauty. But the next day, around lunchtime, I found myself outside the branch of Wolfgang Puck Express in LAX with some time on my hands. I ordered that Caesar Salad. I watched the leaves being turned in their 'signature' Caesar dressing. I saw the plastic bag being opened and the fragments of chicken being poured on. I lifted my plastic fork. I ate. And it all left rather a nasty taste in the mouth.

· Jay Rayner stayed at the Hotel Bel-Air, whose spectacular gardens and individual rooms are a Hollywood favourite. It was recently voted number one hotel in the world for the fourth time. Rooms start at $395 per night. For reservations, contact 001 310 472 1211 (www.hotelbelair.com)