Simmer the mushroom trimmings and a few chopped tomatoes in the vegetable stock to enhance the flavor.
1/2 ounce dried porcini mushrooms ( 1/2 cup)
1 cup boiling water
6 1/2 cups vegetable stock or low-sodium broth
4 tablespoons unsalted butter
2 tablespoons extra-virgin olive oil
1 small onion, minced
1 garlic clove, minced
1/2 pound cremini or shiitake mushrooms, stems trimmed and caps cut into
2 cups arborio rice (14 ounces)
1 1/2 cups dry white wine
1/2 cup freshly grated Parmesan cheese, plus Parmesan shavings for garnish
Salt and freshly ground pepper
In a heatproof bowl, soak the dried porcini mushrooms in the boiling water until softened, about 20 minutes. Rub the mushrooms in the water to remove any grit, then lift them out of the soaking liquid and coarsely chop. Reserve the mushroom soaking liquid.
In a medium saucepan, bring the vegetable stock to a boil. Cover and keep the stock at a bare simmer over low heat. In a large saucepan or medium enameled cast-iron casserole, melt 2 tablespoons of the butter in the olive oil. Add the onion and garlic and cook over moderate heat until softened but not browned, about 4 minutes. Add the cremini mushrooms and cook, stirring, until they are softened, about 4 minutes. Add the rice to the pan and cook, stirring, until coated, about 1 minute.
Add the white wine to the rice and simmer until it has almost evaporated. Add the chopped porcini mushrooms and the reserved mushroom soaking liquid, stopping before you reach the grit at the bottom. Add enough hot stock to just cover the rice and cook, stirring constantly, until the stock has been absorbed. Add more stock. Continue cooking and stirring, adding more stock as it is absorbed, until the rice is just tender and a creamy sauce forms, about 30 minutes.
Remove the porcini risotto from the heat and stir in the remaining 2 tablespoons of butter and the grated Parmesan cheese. Season the risotto with salt and pepper and ladle it into shallow bowls. Top the risotto with Parmesan shavings and serve.