Melt 2 tablespoons butter in a large non-stick pan over a medium heat. Add the rice, stir for a few minutes. Add the onion, stir to combine. Pour about 100ml of the stock over the rice mix, let it be fully absorbed then add another 100ml of stock. Carry on till all the stock has been added. Cover pan with a well-fitting lid, lower the heat, and let it cook for 10-15 minutes.
Meanwhile, melt the remaining butter in a frying pan, and quickly fry the mushrooms.
Add the cooked mushrooms to the rice, stir in the cheese, then the black pepper. Check the seasoning. Finally, add the fresh thyme sprigs and serve.
Best to use butter rather than margarine, to bring out the flavour of the mushrooms. Also, fresh thyme tastes better than dried thyme in this dish. The Parmesan can be substituted for pecorino cheese.