Warm 2 tablespoons butter in a large saucepan over medium-high heat. Stir in mushrooms and cook until soft, approximately 3 minutes. Remove mushrooms and their liquid, and set aside.

Pour stock into the saucepan and bring to a boil, then reduce to a low simmer.

In another saucepan, melt 4 tablespoons butter until foaming. Add onions, stirring frequently. Cook for 2 minutes, then add garlic and cook for 1 additional minute. Cook only until onions are softened, not browned.

Add rice. Stir until a few grains start to pop. Add a few ladles of stock, salt, and pepper. Stir over low heat until the liquid has been absorbed.

Continue adding stock a few ladles at a time, letting the rice absorb the liquid before adding more. Stir constantly. After approximately 20 to 25 minutes, the rice should be just tender and the risotto thick and creamy.

Remove the saucepan from heat. Add parmesan cheese and the remaining 1 tablespoon butter. Mix until the butter has melted.

Stir in mushrooms. Season with more salt, if necessary.

Divide into portions and serve hot. Top with additional parmesan if desired and garnish with a small amount of chopped parsley (optional).