Combine mayonnaise, shallots, mustard, and lemon juice, stirring well. Cover and chill for at least 1 hour or overnight.
Heat a grill pan over medium-high heat. Coat pan with cooking spray. Sprinkle fillets evenly with seasoning. Add fillets to pan; cook 4-5 minutes. Turn over; cook 3-4 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
Place buns, cut-side-down, in pan; cook 1 minute or until toasted. Remove from pan.
Line bottom half of each bun with lettuce, fish fillet, and tomato slices as desired. Spoon 2 teaspoons relish on top of tomatoes. Spread 1 tablespoon mayonnaise mixture over cut side of each top half of buns; place on top of each sandwich.
Recipe adapted from: www.myrecipes.com