Cereal-Crusted Grouper Sandwich

2 to 3 quarts canola or vegetable oil, for frying 3 eggs 2 cups milk Salt and pepper 2 cups all-purpose flour 1 tablespoon Key West seasoning 4 cups sweetened corn cereal, such as Cap'n Crunch Four 8-ounce grouper fillets 1/3 cup tartar sauce, homemade or store-bought 4 large hamburger or brioche buns, split Lettuce, tomato and red onion, for garnish, optional