Sprinkle fish evenly with salt and pepper; set aside.
Spray grate of outdoor grill and utensils with grilling spray. Preheat grill to medium heat. Grill fish 6 to 9 minutes, or until fish flakes easily with fork, turning after 3 minutes.
Mix mayonnaise, relish and onions in small bowl. Spread evenly onto cut sides of rolls. Place lettuce leaves on bottom halves of rolls. Fill rolls with fish and tomato slices.