Heat the butter and oil in a large, heavy-based saucepan. Add the mushrooms and cook for 3 minutes until golden, then remove from the pan and set aside. Add the onion to the pan and cook for 5 minutes until softened, then stir in the garlic. Add the rice and stir until coated in the oil, then pour in the stock.
Bring to the boil, then reduce the heat and simmer for about 15 minutes until most of the liquid has been absorbed. Return the mushrooms to the pan, cover and cook very gently for 5–7 minutes until the rice is tender. Season to taste and stir in the remaining ingredients before serving.