In a saucepan bring water to a boil. Add rice and stir. Reduce heat, cover and simmer for 20 minutes.
Combine onions and butter in a 2 quart, microwave safe, casserole dish. Microwave, uncovered, on HIGH for 20 seconds or until butter melts.
Combine flour, pepper, milk and sherry in a medium mixing bowl; add mushrooms and onion mixture, stir until smooth.
Arrange shrimp around outer edges of the casserole dish. Spoon mushroom mixture into center of casserole. Cover with heavy-duty plastic wrap and microwave at HIGH 5 minutes or until mixture is thickened and shrimp are pink. Stir shrimp and mushroom mixture together. Spoon the mixture over rice. Sprinkle with parsley.
