Heat the oil and half the butter in a large, deep frying pan, add the mushrooms and cook for 2-3 minutes until lightly browned. Set aside. Add the onion to the pan and cook for 5 minutes until softened, then stir in the rice and cook for 2 minutes until the rice is well coated.
Pour the wine into the pan and cook until it has bubbled away. Gradually stir in the hot stock, a little at a time, stirring frequently and allowing the rice to absorb the stock before adding more. When the rice is soft, after about 15 minutes, stir in the remaining butter, the mushrooms and the Parmesan.
Meanwhile, mix together the garlic, grated lemon rind and parsley. Spoon the risotto into serving bowls and scatter over the gremolata.