Cook onions, celery, and green pepper in butter until tender. Add mushrooms, rice, and seasonings. Stir in egg. Turn into a buttered shallow 1-1/2 quart casserole. Cover and bake in 350F for 15 minutes.
Place butter in a 1-1/2 quart microwave proof baking dish. Cook on HIGH (maximum power) 1 minute. Stir in onions, celery, and green pepper. Cook on HIGH 2 minutes. Add remaining ingredients, cover, and cook on HIGH 5 minutes.
Makes 6 servings.