In a medium saucepan, heat stock over low heat and cover.
Heat a very large sauté pan over medium-high heat. Once hot, add mushrooms in a single layer, working in batches if necessary. Heat mushrooms with no oil or salt at first. Cook for 2-4 minutes, or until mushrooms are whistling.
Add in olive oil and thyme, then stir mushrooms. Cook until mushrooms are browned on both sides; stir in salt and pepper. Remove mushrooms from heat and set aside.
In a heavy bottomed stockpot or a dutch oven, heat butter over medium heat until melted. Add in onions and cook for 3-4 minutes or until browned. Add in garlic, stir, and cook until garlic is fragrant (about 30 to 60 seconds).
Add rice to onion mixture. Cook until rice is lightly browned and toasty, about 4 minutes. Pour in white wine and cook until wine is reduced, about 3 minutes.
Add stock into rice mixture about ½ cup at a time, adding more when the stock is almost absorbed into rice, stirring occasionally. (Each addition takes a few minutes to absorb.)
Once the stock is all in the rice and the rice is cooked through, stir in cheese until melted, then remove from heat.
Stir truffle oil into the mushroom mixture. Fold mushrooms into risotto. Serve and garnish, as desired.