1. Fill a deep heavy-bottom skillet with about 2 1/2 inches oil. Place over medium heat until oil registers 365 degrees on a deep-fry thermometer. Meanwhile, combine cornmeal, red-pepper flakes, 1 tablespoon salt, 1 teaspoon pepper, and garlic powder in a shallow bowl or large plate.

2. Season catfish fillets on both sides with remaining tablespoon salt and teaspoon pepper. Dredge fillets in cornmeal mixture, turning to coat both sides. Working in batches so as not to overcrowd the pan, carefully submerge fillets in oil, and fry until crust is crisp and golden and fish is cooked through, about 3 minutes. Transfer to a large plate.

3. To serve, spread bread slices with mayonnaise and/or relish, as desired. Place catfish fillets on half the slices, and top each with a few onion slices and one of the remaining slices of bread.