Preheat the oven to 400 F.
Line your baking dish with foil or parchment paper for an easier clean up.
Slice your mozzarella cheese and tomatoes into thin slices with a sharp knife. You can cut them in half to make them fit into the cuts in the chicken easier. Place the chicken on a cutting board and make 5-6 cuts into each chicken breast. Don't cut all the way through. You want to cut through but leave about 1/4 to 1/2 an inch uncut from the bottom.
Sprinkle the chicken breasts with salt and pepper to taste.
Spread your pesto sauce in between each cut in the chicken breast.
In each cut add a slice of mozzarella cheese and a slice of tomato. If you have fresh basil leaves add one to each slice as well.
Bake the chicken in a 400F oven for about 25 minutes or until the juices run clear and the internal temperature of the chicken is 165 F.
Remove the chicken from the oven and drizzle with balsamic glaze if desired.