Preheat the oven to 450°F. Grease a rimmed 3-quart baking dish (such as a 9 X 13-inch baking dish).
Place the chicken breasts in the prepared baking dish. Spray them with oil and sprinkle them with salt and pepper. Bake them for 20 minutes.
Carefully remove the baking dish from the oven. Spread 2 tablespoons of pesto sauce on 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 the chicken is cooked through (its internal temperature should reach 165ºF) and the cheese is melted, about 5 more minutes. Serve immediately.
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Notes
If your baking dish is broiler-safe, you can finish by broiling for 1-2 minutes to brown the cheese.
The best way to make sure the chicken is cooked through is to use an instant-read thermometer inserted into the thickest part. Aim for an internal temperature of 165ºF.
You can make this recipe with boneless, skinless chicken thighs. They'll cook faster than chicken breasts, so bake them for just 15 minutes instead of 20 minutes at the first cooking stage. After the dish is done, use a thermometer to ensure the chicken is fully cooked.
The leftovers keep okay in the fridge, in a sealed container, for 3-4 days. I prefer not to reheat them so as not to dry the chicken out. I slice them and use them to top a salad the next day. If you'd like to reheat the leftovers, do so gently in the microwave at 50% power.