If you use dried mushrooms, soak them in the (warm) broth while you chop the onion. Heat some olive oil in a pot and fry the onion until lightly golden.
Add the risotto rice, stir for about 1 minute and the mushrooms together with the broth. Add the white whine as well as the thyme branches and let the risotto simmer on medium heat for about 20 minutes, stirring continuously.
Season the risotto with salt and pepper. Take the pot off the heat, take the thyme out of the risotto and stir the butter and the parmesan cheese into it. Serve with fresh parmesan cheese and pepper on top.