In a large pan bring the stock almost to the boil and keep hot on a very low flame.
In another pan melt the butter and sweat the onion until softened but not brown. Add the sliced mushrooms and cook for a few more minutes.
Add the rice, stir well and cook until well coated and slightly translucent.
Add a ladle of hot stock and let it simmer gently until absorbed by the rice. Add another, and so on, until the rice is cooked (15-20 minutes). As a general rule for risottos, you need four times as much water as rice, by volume. And the rice is done when tender but still just a tiny bit chalky in the middle.
Check the seasoning and stir in the olive oil, remaining butter and a couple of spoonfuls of grated parmesan.
Serve at once with the rest of the parmesan on the table.