Add 1 tablespoon of butter to the pot and melt over medium heat. Add the diced shallot and fresh thyme, and cook until the shallot is translucent about 2-3 minutes. Add the remaining 2 cloves of minced garlic, and cook for 30 seconds.
Add the arborio rice to the pot. Stir to coat the rice in the butter and cook for 1 minute until the rice is coated in butter but not browned. Add the white wine to the pot and stir until it has been absorbed by the rice.
Begin adding the warm stock to the pot, one ladleful at a time, stirring constantly and waiting for each addition to be absorbed before adding the next. Continue this process until the rice is cooked through but still slightly firm, about 18-20 minutes.
Stir in the cooked mushrooms, grated Parmesan cheese, remaining butter, and black pepper. Taste and adjust seasoning as needed. Serve hot, garnished with chopped fresh parsley and extra parmesan cheese.