Heat a grill pan over medium-high heat. Coat pan with cooking spray. Sprinkle fillets evenly with seasoning. Add fillets to pan; cook 4 minutes. Turn over; cook 3 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
Place buns, cut sides down, in pan; cook 1 minute or until toasted. Remove from pan.
Combine mayonnaise, shallots, mustard, and juice, stirring well. Line bottom half of each bun with 1 lettuce leaf; top each serving with 1 fillet and 1 tomato slice. Spoon 2 teaspoons relish on top of each tomato. Spread 1 tablespoon mayonnaise mixture over cut side of each top half of buns; place on top of each sandwich.