Preheat the grill or pan to medium heat.
Season the chicken breast with salt and pepper.
Grill the chicken for about 6-7 minutes on each side, or until cooked through.
Remove the chicken from the grill and let it rest for 5 minutes.
Slice the grilled chicken breast into strips.
While the chicken is resting, toast the slices of bread lightly.
Spread the basil pesto on one slice of bread.
If using, spread mayonnaise on the other slice.
Layer the sliced chicken on the pesto side.
Add the lettuce and tomato slices on top of the chicken.
Close the sandwich with the other slice of bread and cut in half.