1. Place brocolli and water in a 2-quart casserole; cover. Microwave until brocolli is fork-tender, 10-12 minutes, stirring after half the cooking time. Drain and set aside.
2. Place mushrooms and butter in a 4-cup measure or medium bowl. Microwave until butter is melted and mushrooms are tender, 2-4 minutes, stirring once. Stir in flour, onion powder, salt and pepper. Blend in milk. Microwave until thickened, 5-8 minutes, stirring once each minute.
3. Pour sauce over brocolli. Sprinkle with grated cheese. Microwave until heated and cheese is melted, another 3-8 minutes.
(*) Using frozen brocolli: Substitute 1 10-oz package frozen cut brocolli. Microwave until fork-tender, 8-11 minutes, stirring after half the cooking time. Continue as directed.