Preheat oven to 350 degrees. Line a small baking sheet with foil and spray or oil the foil. Place two fillets on the sheet and brush them with melted butter. Sprinkle generously with creole seasoning. Top with crushed French's onions and press gently into the fillets.
Bake for 15 minutes, or just until fish flakes apart. While the fish is baking, mix together the mayonnaise, creole seasoning and horseradish. When fish is finished cooking, remove from the oven and serve with lemon and sauce for dipping.
*Yes, that is marinara sauce in the picture. Did it go perfectly with the fish? eh, who cares? It was delicious! I dipped my veggies in it as well and it gave me an excuse to enjoy it again!