Combine mayonnaise, lime juice, chopped mint, lime zest, chilies and sugar in small bowl. Set aside.
Season chicken with salt and pepper. Add chicken and onion to grill and cook 4-5 minutes per side, or until internal temperature reaches 165°F. If desired, add slices of bread last 2-3 minutes to toast.
Spread 2 slices of bread with mayonnaise mixture, dividing evenly. Slice chicken and place on top along with mint leaves, grilled onion and lettuce. Top with remaining 2 bread slices, creating 2 sandwiches.