Preheat oven to 450 degrees F. Grease a baking dish or line it with parchment paper.
Place the chicken breasts in the prepared baking dish. Spray them with oil and sprinkle with salt and pepper. Bake for 20 minutes.
Remove the baking dish from the oven. Spread 2 tablespoons of pesto sauce on the top of each chicken breast. Arrange 2 tomato slices on top, and sprinkle them with 2 tablespoons of shredded mozzarella.
Return the chicken to the oven. Bake until cooked through and cheese is melted, about 5 more minutes. If your baking dish is broiler-safe, you can finish by broiling for 1-2 minutes to brown the cheese.