Combine mayonnaise, lime juice, chopped mint, grated lime peel, chilies and sugar together in small bowl.
Cut onion into ¼-inch thick slices. Grill chicken breast, onion slices and bread over medium-high heat until desired degree of doneness; remove from grill.
Spread 2 slices of bread with mayonnaise mixture, dividing evenly. Slice chicken breast and place on top of mayonnaise mixture along with mint leaves, grilled onion slices and lettuce. Top with remaining 2 bread slices, creating 2 sandwiches.
Send your little ones to school with this nutritious and easy lunch box meal - a Grilled Chicken Mojito Sandwich