I'll admit it: I'm slightly disappointed when Simon Pegg arrives for our lunch and orders a mackerel salad niçoise. The waiter at Brasserie Blanc in Covent Garden gives us a quizzical look. "As a main?" he asks solicitously. Pegg nods. He's been off the booze for a few years now and asks for nothing more than a bottle of sparkling water as accompaniment.

The waiter is appalled. "Nothing else?" Pegg shakes his head.

In person he is lean and compact, dressed in a black T-shirt, with a selection of silver charms on leather strings hanging from his neck. At 43, Pegg takes care of himself – "It's all about nutrition" – and soon he is telling me how he grows his own vegetables at home in Hertfordshire.

"Lettuce, carrots, potatoes …" he lists, eyes gleaming behind Perspex-framed spectacles. "Tomatoes, pumpkins, all the herbs …" When Pegg's daughter Matilda was born four years ago, he and his wife Maureen realised they had "a blank slate. This little life had never eaten an E-number or anything remotely toxic. We had an opportunity to educate her about food."

It's not what I expected from a comedian who has forged a career portraying curry-chomping, pint-swilling suspended manhood. In Spaced, the cult late 90s comedy series he co-wrote and starred in, Pegg played a twentysomething stuck in a prolonged adolescence involving clubbing, casual relationships and sitting around playing computer games in his underpants with his mates. In the subsequent trilogy of high-grossing, critically acclaimed comedy films – starting nine years ago with Shaun of the Dead, moving on to Hot Fuzz in 2007 and reaching its culmination earlier this year with The World's End – Pegg wrote a succession of brilliant parts for desperate men who have never quite outgrown their childhood. Pegg starred in all three films with his best friend, Nick Frost.

"The man-child is a modern phenomenon which came about because the pressure on us to conform with marriage and children was lifted," Pegg explains. "Our parents got married at 22, 23 but by the time I reached that age, it was less incumbent on me to conform. Spaced came about from that: without a new set of criteria between 20 and 30, you fill your life with stuff you did as a kid – video games, comics, juvenile pursuits. It does create a reluctance to let stuff go, particularly with guys who are the children of the 70s and who were brought up with Star Wars. You also struggle to face commitment. I found when I finally got married and had the kid, that's when the world made more sense to me. It was, 'Oh, I see, that's what we're there for.'"

Pegg unabashedly describes Frost as his "rock". The two have been friends for 20 years, ever since Pegg's former girlfriend introduced them (she and Frost were waiters at the same north London Mexican restaurant). Pegg says he wanted Frost to be in Spaced simply so he could have an excuse to hang out with him more. For a while, the two of them were famously so strapped for cash that they shared a single bed. What was that like?

"Cramped," Pegg says, unfolding his napkin. "I'd recommend it. One of the worst things in the world is homophobia. There are a lot of guys who are worried they might be gay and I'd say a) don't worry about it. If you're gay, you'll end up having loads of fun and b) if you get into bed with your best mate and you don't want to have sex with him, you're not gay!"

In a satirical homage to Krzysztof Kieslow´ski's Three Colours film trilogy, Frost and Pegg decided to call their films the Three Flavours Cornetto Trilogy (a Cornetto appears at some point in each film). Does Pegg have a favourite Cornetto? "Original," he says, sipping on iced water.

The salads arrive. I have been shamed into ordering something equally frugal involving broad beans. We munch our foliage dutifully.

To be fair to Pegg, he is here to promote a healthy-eating food app for children so it would be wrong for him to gorge himself on chicken nuggets. Henri le Worm is aimed at kids between two and 10 years old and features lots of games, recipes and interactive songs. It was developed with the help of Raymond Blanc and Pegg is providing Henri's voice. "The tough thing with kids is that mealtimes are quite boring," he says. "They're always diverted by something visually stimulating like a character like Henri."

As a child growing up in the Gloucestershire village of Brockworth, Pegg was never given junk food. But he can remember sampling his first McDonald's milkshake in London in 1983 and thinking it was amazing.

When he was seven, his parents divorced. His mother remarried and Pegg took on his stepfather's surname. He always loved performing and was encouraged by his mother, Gillian, a stalwart of the local am-dram scene.

In the past, Pegg has attributed his ambition to the feeling of being dumped by his father. (They now have a good relationship). Perhaps, as the eldest of four, he bore the brunt of the emotional fallout and acting became a means of socking it to the world – none of his siblings has become anything remotely approaching a movie star. We start talking about how difficult it must be when the brother of a famous film star is also an actor but has never quite hit the big time. Like Eric Roberts, I say, the elder brother of the more famous Julia.

"I used to travel to New Mexico every weekend with Eric Roberts," Pegg says, reaching for the salt and grinding copious amounts on to his mackerel fillet. "He's a really nice guy."

Later, when I ask about the annual cheese-rolling contest that takes place in his home town, Pegg launches into an anecdote about seeing a clip of it on American television and realising for the first time how "insane" it was. "I was in the Chateau Marmont," he adds.

These are the kind of things you can only say when you've become a bit Hollywood. Pegg has won parts in blockbusters including Mission: Impossible III (alongside Tom Cruise), Tintin (directed by Steven Spielberg) and JJ Abrams's lucrative Star Trek franchise. But when I ask him about his favourite films, he names the Coen Brothers' Raising Arizona and Martin Scorsese's Taxi Driver, as well as Back to the Future and Gremlins, both of which he admires for being "beautifully structured".

He remains healthily aware that the hype of celebrity is "mostly bullshit", and balances it out by spending as much time as he can with his wife and daughter and their two dogs, Minnie and Myrtle. Pegg has an "M" tattooed on his right wrist. Does everyone in his life have a name starting with that letter? He grins, "Yeah."

His mother got a dog recently. "She called him Bertie…"

Did she not get the memo? I joke.

"…after her Dad," Pegg concludes.

Oh. He has the grace to laugh.

Pegg is more serious and sincere than I had anticipated. There's plainly a lot going on in his head. As he clears his plate of the final frond of lettuce and orders a double espresso, he treats me to a Marxist deconstruction of Star Wars – it was the topic of his dissertation at Bristol University – which involves a theory about the "infantilisation of cinema, arguably as a means of control, of keeping us in a state of arrested development".

So how does he square this with his role in Star Trek? "If you can't beat 'em, join 'em," he says and flashes a mischievous grin that transforms his face. "I'm a massive hypocrite."

Henri le Worm: The Missing Cookbook (iTunes, £2.99) is available for iPad and iPad mini; henrileworm.com