Season the chicken with salt and pepper. Melt the butter in a flameproof casserole, then add the chicken and gently fry for 5 minutes until lightly browned. Add the onion and cook for a further 5 minutes. Add the garlic and rice and cook for a further 1 minute, stirring.
Pour in the wine and let the mixture bubble until the wine has almost evaporated. Stir in the stock and saffron and bring to the boil.
Cover with a lid and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10 minutes until the stock is absorbed and the rice is almost tender.
Stir in the mascarpone, tarragon, parsley and mangetout or peas. Mix well until the cheese has melted, then cover and return to the oven for a further 5 minutes. Stir in a little boiling water if the mixture has dried out. Check the seasoning and serve with a leafy salad.