Put the stock in a saucepan and bring to a very gentle simmer.
Meanwhile, melt the butter in a saucepan, add the onion, garlic and salt and pepper and cook over a low heat, stirring occasionally, for 10 minutes until the onion is softened but not browned. Add the rice and cook, stirring, for 1 minute until all the grains are glossy. Stir in the wine, bring to the boil and continue to boil for 1–2 minutes until absorbed. Stir in the peas.
Stir about 150 ml (¼ pint) of the stock into the rice. Cook over a medium heat, stirring constantly, until absorbed. Continue to add the stock, a little at a time, and cook, stirring constantly, for about 20 minutes until the rice is al dente and the stock has all been absorbed.
Remove the pan from the heat. Stir in the mint and Brie, cover and leave to stand for 5 minutes until the cheese has melted. Serve with grated Parmesan.