Preheat oven to 400 degrees. Spray a 1 1/2-quart casserole with nonstick cooking spray. In a large skillet over medium heat, cook onion and mushrooms in butter until soft. Sprinkle with flour, salt, and pepper. Add half-and-half and sherry. Cook until thickened. Add chicken, ham, and water chestnuts. Pour mixture into casserole. Top with cheese. Combine crumbs and melted butter and sprinkle over top. Bake for 25 minutes or until bubbly and golden. This dish freezes well.