In a large, heavy-bottomed pot, skillet, or Dutch oven, heat 2 tablespoons of butter and the olive oil over medium-high heat.
Add the sliced mushrooms and season with salt and pepper. Cook until the mushrooms have released their liquid and start sweating, about 3-4 minutes. Add half of the garlic, cook for 30 seconds then remove from pot and set aside.
Add 1 tablespoon of butter to the pot and melt over medium heat. Add the diced shallot or onion and fresh thyme, and cook until the shallot/onion 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 (make sure that the rice does not stick to the bottom) 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 (al dente), 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.