Melt the butter in a large deep pan over a medium heat. Add the onion and cook for 5 minutes, until softened. Add the mushrooms and cook for a further 5 minutes, until softened.
Add the rice and cook for another 5 minutes, stirring, before you add the stock. Reduce the heat to low, cover, and leave to cook for 20 minutes, or until all the liquid is absorbed.
Stir through the cheese and season with freshly ground black pepper. Divide between bowls and serve.