Add dried shiitakes to a high-speed blender and blend until a fine powder. Let the powder settle before opening the blender lid. Add 4 cups of stock and blend on low speed until powder is incorporated. Transfer to a medium saucepan and add the remaining 3 cups of vegetable stock. Cover with a lid and bring to a simmer over medium heat. Set aside a ladle for transferring stock to the risotto.

Melt 1 tablespoon butter in a large, wide heavy pot over medium heat. Add onion and cook until translucent, about 3 minutes. Add garlic and cook until fragrant, about 1 minute. Add rice and cook for 1 minute, stirring to coat.

Ladle about 1½ cups of hot stock over the rice. Stir in ½ teaspoon of the salt and the pepper. Cook, stirring frequently, to prevent the rice from sticking. When the broth is almost absorbed, add about 1 cup of broth at a time and repeat. Cook until the rice is tender and the mixture is creamy, 16 to 22 minutes.

In a skillet, melt 1 tablespoon of the butter. Add mushrooms and thyme. Cook until mushrooms are golden brown, 5 to 8 minutes. Stir in the remaining ¼ teaspoon of the salt. Remove from heat.

Gently stir the remaining 1 tablespoon of butter and Parmesan into the rice. Taste and adjust seasonings if needed. Spoon risotto into dishes, top with mushrooms, and garnish with parsley and more cheese.