Put the mushrooms in a bowl, pour 425ml boiling water over and leave to soak for 10 mins. Meanwhile, heat the oil in an ovenproof pan and fry the onion for 2 mins until starting to soften. Add the garlic and cook for another min.
Heat oven to 190C/fan 170C/gas 5. Drain the mushrooms, reserving the liquid, and chop. Add the mushrooms, thyme and rice to the pan, then stir well. Strain over the mushroom liquid, pour in the stock and wine and bring to the boil.
Season to taste, cover and bake for 25 mins or until the rice is just cooked and all the liquid has been absorbed. Stir in the grated Parmesan, check the seasoning and sprinkle with extra thyme leaves and Parmesan shavings to serve.