Bring 1 l of stock to a near boil in a pan and keep it hot over a low flame.
Melt some butter in another pan and sweat the chopped onion until softened.
Add the sliced mushrooms to the onions and cook for several minutes.
Stir in the rice and cook until the grains are well coated and slightly translucent.
Add one ladle of hot stock and simmer gently until the liquid is absorbed by the rice.
Continue adding stock gradually for 15 to 20 minutes until the rice is tender but firm.
Season the mixture and stir in the olive oil, remaining butter, and some grated parmesan.
Serve immediately with the remaining parmesan cheese on the side.