To make mushroom broth: In a stockpot, combine the stock, crimini mushrooms, and dried mushrooms. Bring to a boil over high heat, reduce the heat, and simmer, covered, for about 30 minutes, until the flavors have infused. Strain through a fine-mesh sieve, pressing on the solids to extract as much flavor as possible. Discard the solids. Return the strained stock to the pot, cover, season with salt and pepper, and keep at a simmer over low heat.
To make risotto: In a large saute pan, heat 1 tablespoon of the butter over high heat. Add the mushrooms and season with salt and pepper. Cook, stirring occasionally, for 1 to 2 minutes, or until the mushrooms have softened. Set aside.
In a large stockpot, heat 2 more tablespoons of the butter and the olive oil over medium-high heat until the butter melts. Saute the shallots for 1 to 2 minutes, until softened. Add the garlic and cook for about 2 minutes longer. Add the rice and cook, stirring with a wooden spoon, for 7 to 10 minutes, until it turns milky white and opaque, and begins to stick to the bottom of the pan. Add the thyme and continue stirring.
Add the wine and stir for about 2 minutes, until nearly absorbed. Ladle about 1 cup of the simmering stock into the rice. Cook for about 2 minutes, stirring often, until the stock is almost completely absorbed. Add more stock, a cup at a time, stirring gently, until the broth is absorbed by the rice before adding the next cup. After about 15 minutes, begin tasting the rice. At this point, add the stock judiciously. The rice should be firm, yet cooked through in 18 to 20 minutes total cooking time.
Stir in the sauteed mushrooms, the Parmesan, and the remaining 2 tablespoons of butter. Season with salt and pepper, and serve.
Recipe courtesy of Alfred Portale's Twelve Seasons Cookbook (Broadway, 2000)