Stir together the dried porcini and 1 cup of the chicken broth in a microwave-safe 2-cup or larger measuring cup. Microwave, uncovered, on high (at 100 percent power) until the broth is hot but not boiling, 1 1/2 minutes in an 1,100-watt oven or 2 1/2 minutes in a 700-watt oven. Let sit for 5 minutes. Remove the porcini pieces from the liquid with a slotted spoon, squeezing out excess moisture in the process. Chop the porcini coarsely. Reserve the chopped porcini and the porcini broth separately.
Place the shallots, garlic, cremini and shiitake mushrooms, 2 tablespoons of the butter and 1/4 teaspoon salt in a microwave-safe 8-by-8-by-2-inch baking dish. Cover tightly with 2 pieces of plastic wrap; make a small slit in the center with the tip of a paring knife to vent excess steam. Microwave on high (at 100 percent power) until all the vegetables have softened, 5 minutes in an 1,100-watt oven or 9 minutes in a 700-watt oven. (When removing the plastic, be careful to avoid the hot steam.) If some of the vegetables still seem a little raw, stir and microwave again, covered, in 30-second increments.
Add the rice to the baking dish, and stir to coat the grains. Add the white wine, 1 1/2 cups of the remaining chicken broth, reserved porcini broth (leaving the sediment at the bottom of the cup) and chopped porcini, thyme and 1/2 teaspoon salt. Cover and microwave on high (at 100 percent power) for 8 minutes in an 1,100-watt oven or 10 minutes in a 700-watt oven. Stir the risotto, and add the remaining 1 cup chicken broth. Cover and microwave on high (at 100 percent power) until the rice is cooked but with the faintest bite in the center, 9 minutes in an 1,100-watt oven or 15 minutes in a 700-watt oven. Add the remaining 2 tablespoons butter, Parmesan and parsley, and stir vigorously until creamy. At this stage, the risotto should be thick but pourable from a spoon; if it's too stiff, splash in more broth. Let sit, covered, for 2 minutes to rest and come together. Serve with a sprinkling of grated Parmesan on top.
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