Listen to Michel Roux Jr and his sous chef Monica Galetti talk about their history, and it's a wonder they are still on speaking terms, let alone regard each other as mentor and protege. And yet anybody who watched Galetti on BBC2's MasterChef: The Professionals, judging the skills test to decide which contestants were good enough to go before her boss, will know just how devoted to him she is. Her intense stare, her inability to hide even for a second her disapproval on her intensely mobile face, turned Galetti into a cult figure. In an often saccharine TV food culture that forces its personalities to smooth every rough edge, her refusal to compromise and her grinding authenticity were hugely refreshing. Not to mention entertaining. "I am always deadly serious about it," Galetti says. "I know it's only a TV programme but when I am judging them I always have Michel in mind. It's vital for me that he has as good a quality of cooks before him as possible."

She has worked for Roux at Le Gavroche, the most gloriously classical of London's high-end restaurants, since 2000. "For me spotting talent and bringing it on is one of the most important parts of the job," says Roux, who inherited custodianship of the restaurant from his father, Albert. So how did he spot Galetti? "She sent in a CV and it must have stood out, because I receive a dozen a day. Plus she was based in New Zealand and I couldn't just ask her over for a trial. It had to be a job or nothing."

Galetti, now 35, was born in Western Samoa but trained as a chef in New Zealand. While Pacific Rim and fusion cooking were very much in vogue she says she always had her eyes on one of Britain's great classical restaurants. "When I was growing up, the Roux brothers, Michel's father and uncle, Albert and Michel Sr, they were my idols," Galetti says. "I regarded them as royalty." She applied to a number of kitchens before she landed the job as a very junior cook at Le Gavroche.

So was it all she hoped for? She rolls her eyes. "No! Michel was so hard on me. Early on I was running the starters section and I remember him shouting at me: 'If you can't do the job just get on the next plane back to New Zealand.'" Roux has exactly the same memory. "I did keep telling her to go home. But the thing is I tend to be more exacting on the ones who are worth bothering with. And even from early doors I could see she was very driven."

Both agree the turning point in their relationship came courtesy of a tragedy in Monica's life, after a couple of years in Britain. "She had a sudden bereavement and had to go home," Roux says. "She told me she'd be back but I thought: 'That's the last I'll see of her.'" After all, Galetti had already tried to quit a couple of times, exhausted by the knockbacks and the hours. "But after eight or nine months she called and said she was ready to come back. That showed real commitment." Galetti agrees: "When I returned I just felt I had nothing to lose." Galetti says Roux was "still a hard arse", but the kind she could relate to. "He's a very good teacher, very encouraging and very patient. Once you get over your fear here, you realise there is so much to learn." Having worked every section she is now one of two sous chefs, beneath head chef Rachel Humphries, and often runs the kitchen during service. Roux rhapsodises about Galetti's palate and her attention to detail, her work ethic and her professionalism. So how did she land the role in MasterChef? "I suggested a few people to the producers," Roux says, "but they immediately went for her. She gets teased that she's Miss Nasty but really she's just a pro. She's a complete darling."

One question hangs over them both, though: where does the complete darling go next? She agrees that her life is completely interwoven with Le Gavroche. Her husband, David Galetti, is the head sommelier; as she herself says, meeting other people is almost impossible, given the working hours. They now have a daughter, and Monica works part time, although that is part time Le Gavroche style: she still does a 40-hour week.

"Maybe David and I will eventually have a place of our own but I think Michel will have some involvement with whatever we do in the future." Roux agrees: 'I'm greedy. I want to hang on to her for ever. But it is part of the history of Le Gavroche that we always help our top chefs with their future plans." MasterChef at Home is out now, published by Dorling Kindersley, £20