Combine onions and butter in a 1 1/2-quart casserole.
Arrange shrimp over onions around outer edge of casserole.
Place mushrooms in center of shrimp; sprinkle with flour, salt, and pepper.
Cover with heavy-duty plastic wrap and microwave at HIGH for 4 minutes.
Cover and microwave at HIGH an additional 2 minutes.
Stir in sour cream and lemon juice.
Microwave, uncovered, at HIGH for 35 to 40 seconds or just until thoroughly heated (do not boil).
Serve over rice; sprinkle with fresh parsley