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 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.