Combine first 4 ingredients in a shallow dish. Dredge fish in flour mixture, shaking off excess.
Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add fish to pan; cook 3 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, combine mayonnaise, relish, and red pepper. Spread about 1 teaspoon mayonnaise mixture over cut side of each roll half. Layer 2 tomato slices, 1 onion slice, and 1 fillet on bottom of each roll; top with top halves of rolls.