Heat 2 tablespoons of the oil in a large non-stick pan. Add the onions and fry over a medium heat for 5 minutes to soften them. Stir in the rice, mix well and cook for a further 2 minutes.
Reduce heat to medium low and start pouring in the stock, about 250ml at a time. Add more stock as each addition is absorbed. Continue in this manner for about 20 minutes until the rice is cooked al dente and the stock is absorbed. (All the stock may not be needed.)
Whilst the risotto rice is cooking, heat the remaining tablespoon of oil in a large non-stick frying pan and fry mushrooms on a high heat for about 5 minutes until golden. Season well and set aside.
Stir the cooked mushrooms into the rice along with the butter and chopped chicken. Heat through gently and season to taste.