Cut 4 large sheets of heavy-duty foil. Spray sides with cooking spray.
In the chicken breasts, cut 4 crosswise slits about 3/4-inch apart, carefully cutting almost to bottom of chicken breast, but not all the way through. Place each chicken breast on a piece of foil.
Spoon 1 teaspoon pesto into each slit in chicken breast. Place 1 mozzarella slice and 1/2 tomato slice in each slit. Sprinkle with Italian seasoning, and salt and pepper.
Bring up 2 sides of foil, sealing the edges; do the same on the other side, making a sealed packet.
Place packs on grill over medium heat. Cover the grill; cook 10 minutes. Rotate packs 1/2 turn; cooking 9 to 10 minutes longer. Check to see that chicken is done in the center.
Carefully fold back the foil; garnish with fresh basil and serve!