Soak the porcini mushrooms in 100ml of boiling water for 15 minutes. Remove mushrooms from the soaking water and drain well, reserving the soaking liquid. Coarsely chop the porcini and strain the soaking liquid into a saucepan. Add enough vegetable stock to make up 1 litre and bring to the boil over medium heat. Reduce the heat to maintain a low simmer.
Meanwhile, melt half of the butter in a large, deep-frying pan (or stockpot) over medium heat. Add the onion and cook without browning for 2 minutes or until soft. Add the mushrooms and porcini. Cook, stirring, for a few minutes.
Stir in the rice and after 2 minutes, pour in the wine. Season with sea salt to taste. Cook, stirring, until all the liquid has been absorbed.
Add a ladleful of the hot stock, stirring constantly. When the stock has been absorbed, stir in another ladleful. Continue to add ladleful’s of stock, stirring and allowing each ladleful to be absorbed before adding the next until the rice is ‘al dente’ (approximately 15 minutes).
Remove the frying pan (or stockpot) from the heat. Stir in the remaining butter and Parmigiano Reggiano. Serve the risotto of dried porcini and mushrooms with grated Parmigiano Reggiano on top – buon appetito.