1. Preheat oven to 450°. Place baking sheet in oven.
2. Combine mayonnaise, relish, capers, 1/8 teaspoon salt, and pepper sauce in a small bowl, stirring well. Set aside.
3. Combine flour, paprika, 1/2 teaspoon garlic powder, 1/8 teaspoon salt, and pepper in a shallow dish. Place egg whites in a shallow bowl. Combine cornmeal, remaining 1/8 teaspoon salt, and remaining 1/4 teaspoon garlic powder in a shallow dish. Working with 1 fillet at a time, dredge in flour. Dip in egg whites; dredge in cornmeal mixture. Place fillet on a plate; repeat procedure with remaining fillets, flour, egg whites, and cornmeal mixture.
4. Transfer fillets to preheated baking sheet coated with cooking spray. Lightly coat fillets with cooking spray. Bake at 450° for 6 minutes. Turn fillets and coat with cooking spray; bake an additional 6 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
5. Place 1 bottom half of roll on each of 4 plates. Top each serving with 1/4 cup lettuce, 1 fillet, 2 tomato slices, and 1 onion slice. Spread 4 teaspoons mayonnaise mixture on cut side of each roll top; place on top of sandwiches. Serve immediately.
Beer note: Long before the term "pilsner" was appropriated by American megabrewers, it described a lager with floral aromatics and a clean, dry finish. Stoudt's Pils ($8/6-pack) from Pennsylvania revives that tradition, making this beer ideal with lighter fare like these sandwiches. This pilsner is crisp, yet flavorful, with a frothy white head, bready aromas, and satisfying hop bitterness that cuts through the creamy mayonnaise. —Jeffery Lindenmuth