Preheat the grill to medium-high heat. Grill the chicken breasts for 12 to 15 minutes, or until no pink remains, turning them over halfway through the grilling. Spoon about 1 tablespoon pesto sauce evenly over the top of each piece of cooked chicken. Place a slice of mozzarella cheese over the pesto sauce, close the grill cover, and continue cooking for 2 to 3 minutes, or until the cheese is melted.
Pesto sauce is usually available in the supermarket dairy case near the fresh pasta.