Chop onion and saute in the olive oil for 2-3 minutes. Add rice and stir for a couple minutes until rice is well saturated. Ladle in the hot stock 1/2 cup at a time, stirring each time until stock is absorbed before adding more. Add some mushrooms each time. Stir and ladle more stock until rice is done. About 30 minutes. It should be chewy, not soupy. Stir in Parmesan Cheese, serve hot.