Chop the onion and let it brown in a pot with a drizle of oil. Add the rice and let it toast, simmering with white wine; wet with stock and cook. Meanwhile, in a pan cook the mushrooms with the boiled chestnuts cut into pieces. Once ready, keep them aside, and add them to the rice 5 minutes before the end of cooking: finally, over a low heat, stir the risotto with butter and Parmesan cheese. Garnish with fresh thyme and serve when it’s still hot.