Saute first 3 ingredients in a large nonstick skillet coated with vegetable cooking spray over medium heat 5 minutes or until tender. Add mushrooms, and saute 2 minutes. Stir in rice, next 4 ingredients, and, if desired, sherry; spoon into a lightly greased 13- x 9-inch baking dish. Top with chicken. Sprinkle chicken with salt and pepper.
Bake, covered, at 350° for 1 hour or until chicken is done.