Soak the porcini mushrooms in hot water for 10 minutes, then drain well. Heat the oil in a large, heavy based saucepan and add the onion and garlic. Fry over a gentle heat for 2-3 minutes, until softened. Add the chestnut mushrooms and fry for a further 2-3 minutes, until browned.
Stir in the rice and coat the oil. Pour in the wine and simmer, stirring again, until the liquid has been absorbed. Continue adding the stock in this way, until all the liquid has been absorbed and the rice is plump and tender.
Roughly chop the soaked porcini mushrooms and stir into the risotto, along with the parsley, butter, and salt and pepper.
Serve with freshly grated Parmesan.
Watch the video below for additional tips and tricks.