Juice half of the lemon. Cut remaining half into wedges. For sauce, in bowl combine juice, mayonnaise, shallot, relish, and mustard. Season with black pepper.
Lightly season flounder with kosher salt. Sprinkle flour onto a large plate. Dip fish into flour, shaking off excess. In a large skillet heat oil over medium-high heat. Carefully add fish to skillet (do not crowd; cook in batches if necessary). Cook 4 to 6 minutes per half-inch thickness or until fish flakes easily with a fork, turning once.
Spread sauce on each bread slice; sprinkle with half the herbs. Top with fish and remaining herbs. Serve with lemon wedges.