Start by heating your vegetable stock in a separate pot until it just simmers; keep it warm on low heat throughout cooking.
Slice your mixed mushrooms into bite-sized pieces, then set aside. Finely dice the shallots and garlic for sautéing.
Heat the olive oil in a wide, heavy-bottomed pan over medium heat. Add the mushrooms and a pinch of salt, sautéing until they release their juices and turn deep brown, about 8 minutes. You should hear a gentle crackle and see the mushrooms develop a rich, earthy aroma.
Push the mushrooms to the side of the pan, then add the diced shallots and garlic. Cook gently until translucent and fragrant, about 2 minutes. They should smell sweet and aromatic, with a slight shimmer of oil.
Add the Arborio rice to the pan, stirring constantly for about 2 minutes until the edges look translucent and the rice starts to crackle. This coats the grains with oil and begins the starch release process.
Pour in the white wine, stirring continuously until the wine mostly evaporates and the pan smells bright and slightly tangy, about 1-2 minutes.
Begin adding the hot vegetable stock one ladleful at a time, stirring constantly. Wait until most of the liquid is absorbed before adding the next ladle. Maintain a gentle simmer, and repeat this process for about 18-20 minutes, until the rice is tender but still has a slight bite and the mixture is creamy and glossy.
Once the rice is cooked to your liking, turn off the heat and stir in the butter, grated Parmesan, and a squeeze of lemon juice. Taste and adjust seasoning with salt and pepper. Let it rest for 2 minutes to allow flavors to meld.
Garnish with fresh thyme and additional Parmesan if desired. Serve the mushroom risotto hot, enjoying its rich, earthy aroma and creamy texture with tender browned mushrooms throughout.
