In bowl, stir together mayonnaise, onion, tomatoes, relish and cayenne pepper. Set aside.
Season both sides of fillets w/ old bay seasoning and garlic powder. Melt butter in skillet over medium heat. Add fillets and cook about 3-5 min per side, or until fish flakes easily and is well browned.
Place 2 slices of cheese on each fillet. Cover skillet until cheese melts. Place each fillet onto a slice of toast. Spread w/ the mayonnaise sauce. Top w/ remaining toast.