Preheat oven to 375°F. Combine topping ingredients and set aside.
Cook onion in butter in a large skillet over medium heat. Add celery, mushrooms, and seasoning and cook an additional 5-7 minutes or until tender crisp.
Stir in water chestnuts and soup.
Place in a 1 ½ qt casserole dish and top with prepared topping.
Bake 20-22 minutes or until browned and bubbly.
