Mix together the soy sauce, mirin, sesame oil, ginger and Sriracha.
Toss the shiitakes and radishes in a microwave-safe 8-by-8-by-2-inch baking dish. Place the fillets on top in a single layer. Pour the soy sauce mixture over the fillets. Cover tightly with 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 the fillets are opaque white, 4 minutes in an 1,100-watt oven or 6 minutes in a 700-watt oven. Let stand, covered, for 2 minutes to steam and finish cooking gently.
Meanwhile, place the snow peas in a microwave-safe medium bowl. Cover tightly with plastic wrap; make a small slit in the center. Microwave on high (at 100 percent power) until the snow peas have turned bright green but are still crisp, 1 1/2 minutes in an 1,100-watt oven or 2 1/2 minutes in a 700-watt oven. (When removing the plastic wrap, be careful to avoid the hot steam.)
Divide the snow peas between two plates. Top with the shiitakes, radishes and fillets. Drizzle any sauce left in the baking dish over the fish. Squeeze fresh lemon juice over the fillets, and garnish with scallions. Serve with brown rice.
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