Soak the dried mushrooms in 500ml hot water for 30 minutes. After 15 minutes, start cooking the risotto.
Heat oil in a large pan, add the onion and gently fry for 5 minutes. Add the garlic and cook for 3 minutes more.
Stir in the rice and thyme, fry for 2-3 minutes, and stir until rice is translucent.
Increase the heat slightly and stir in the wine, allow to bubble off for a minute. Stir in the dried mushrooms along with their soaking liquid. Add the stock a ladleful at a time, stirring as you go, allowing the liquid to be absorbed before adding the next. After 10 minutes, stir in the chestnut mushrooms and cook for a further 5-6 minutes until the rice is just al dente.
Turn off heat, stir in the parmesan and cover with a lid. Set aside for 2 minutes. Season with black pepper and serve.