Soak the mushrooms (boletus edulis, called funghi porcini in italian) in a non-metal bowl of warm water for about 30 min. Meanwhile, melt the butter in a pot, add the onion and fry until soft. Add the rice (if you can't find vialone use any other round-grained type) and cook for 1-2 minutes while stirring. Add the wine and allow it to boil until almost all has evaporated. Then add the stock in portions with a ladle. Add each portion when the last has been almost completely absorbed by the rice. Dissolve the saffron in the last portion and add it to the rice. Add the soaked mushrooms (cut to bite-sized pieces). If the rice is not soft yet, you might need to add some more stock. Just before serving stir in the butter and if you wish, Parmesan cheese.