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 saucepan and bring to boil, then reduce to low simmer. Ladle a small amount of stock into a small bowl. Add saffron threads and let saffron infuse stock.
In another saucepan, melt 4 tablespoons butter until foaming. Add onions, stirring frequently. Cook 2 minutes, then add garlic and cook 1 additional minute. Cook only until onions are softened, not browned.
Add rice. Stir until a few grains start to pop. Add saffron liquid, a few ladles of stock, salt, and pepper. Stir over low heat until 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, rice should be just tender and risotto thick and creamy.
Remove saucepan from heat. Add parmesan cheese and remaining 1 tablespoon butter. Mix until 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).