Preheat oven to 200°C/400°F/gas 6. Place the mushrooms in a bowl and cover with 180ml of boiling water. Set aside to soak for 5 minutes, then drain, reserving the soaking liquid.
When they are soaked, chop the porcini and place in a 3-litre casserole dish with the reserved soaking liquid. Trim and chop the chicken thighs, then peel and crush the garlic. Add both to the casserole dish along with the rice and stock. Pick in most of the thyme leaves, reserving a few for garnish. Stir everything to combine, cover tightly with tinfoil and cook for 35 to 40 minutes, until the rice is cooked.
Meanwhile, chop up the butter and finely grate the Parmesan. When the risotto is ready, remove from the oven and add the butter and cheese to the risotto. Season with sea salt and black pepper, then stir for 3 to 4 minutes, or until the risotto thickens. Divide between bowls and top with extra Parmesan to serve.