Soak the porcini mushrooms in hot water for 10 minutes, then drain well. Heat the oil in a saucepan and add the onion and garlic. Fry over a gentle heat until softened. Add the mushrooms and the ½ glass of wine and fry until browned.
Stir in the rice and coat in the oil. Pour in the wine and simmer, stirring, until the liquid has been absorbed. Add a ladleful of the stock 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.
Add the parsley, butter and, if necessary, salt and pepper. Serve with freshly grated Parmesan.