Preheat oven to 400°F.
In a large oven-proof pan, add olive oil. Enough to just cover the bottom of the pan. Depending on how big your pan is, it should be about 2-3 tablespoons.
Turn on heat to medium-high. Add chicken breasts. Lightly salt and pepper the tops of the chicken. Also add 1 cup of water, cover chicken, and let cook for 8-10 minutes until cooked through.
Once chicken is cooked, use side of a wood spatula to cut the chicken into smaller pieces, about 1 inch in size. (You can also cut the chicken up before cooking it but I find this to be cleaner and achieves the same result.) Cutting up the chicken before you put the pesto in allows the pesto flavor to thoroughly coat each piece of chicken, and each bite.
Sautee chicken for 1-2 more minutes, until well cooked but tender. Turn off heat.
To pan, and 1-1.5 cups of pre-made pesto. Mix well to ensure all the chicken is coated with the pesto.
To the top of chicken, add slices of the fresh mozzarella, alternating with the slices of tomato. Cover the top and place into the preheated oven.
Bake for about 15-20 minutes until the cheese is melted and slightly browning, and the tomatoes are cooked.
Remove from the oven (be sure to use an oven mitt - it will be hot!), and top with fresh basil and if desired lemon zest (or fresh lemon juice).
This dish is great on its own or served with pasta, or greens, or even rice or potatoes. Hope you love this One Pan Pesto Chicken!