Preheat the oven to 400°F (200°C).
Line a baking sheet with foil and lightly coat the foil with cooking spray.
In a mixing bowl, toss the chicken breasts with the 1/2 cup basil pesto until evenly coated.
Arrange the pesto-coated chicken breasts on the prepared baking sheet in a single layer.
Bake the chicken for 35 minutes, or until the internal temperature reaches 165°F (74°C) and the chicken is cooked through.
Remove the baking sheet from the oven. Top each chicken breast with sliced tomatoes (if using) and sprinkle evenly with the shredded mozzarella.
Return the baking sheet to the oven and bake 5–7 more minutes, until the cheese is melted and bubbly. Serve warm.
