Gently heat the oil in a large frying pan. Add onion and sauté for 2 - 3 minutes until softened then add the mushrooms and garlic and sauté for a further 2 - 3 minutes until mushrooms are browned.
Empty the contents of the saucepan into a large oven-proof baking dish, add the rice and mix well together.
Place the chicken breasts on the rice and season well with the salt and pepper.
Pour the chicken stock all-round the dish.
Cover with foil and bake for 1 hour until most of the liquid is absorbed.
Once cooked remove from the oven, stir in the frozen peas, and replace the foil on top allowing the casserole to sit for about 5 minutes.
Serve each chicken breast on a bed of the rice, onion, mushroom, and pea mixture.