Stir together the mayonnaise, onion, tomato, relish and piri piri in a small bowl. Season to taste with salt and pepper and set aside.
Season the whiting fillets on both sides with the seafood seasoning and garlic powder. Melt the butter in a frypan over medium heat. Cook the fillets until they are golden brown on both sides and no longer translucent in the center, 2 to 3 minutes per side. Place two slices of cheese on top of each fillet, cover the frypan, and cook until the cheese has melted.
Place each fillet onto a slice of toasted bread, spread with the reserved sauce, then top with the remaining slices of bread.
If you can't find piri piri, use cayenne pepper instead.