Cornflake Chicken Casserole

Ingredients (8)

• + 2 (10.5 ounce) cans cream of chicken soup add to pantry
• + 3 cups chopped cooked chicken, or more to taste add to pantry
• + 1 cup finely chopped celery add to pantry
• ✓ 1 cup cornflakes cereal
• + 1 (8 ounce) can sliced water chestnuts, drained add to pantry
• + ½ cup mayonnaise add to pantry
• + ½ cup finely chopped onion add to pantry
• + 2 tablespoons butter, melted, or as needed add to pantry