Preheat the oven to 425ºF.
Prepare the chicken breasts. Start by cutting them in half lengthwise, so that you get two super thin, long, large slices of chicken breast. Tip: make sure to use a sharp knife.
Using the meat pounder, pound them gently until they are even thinner, but don't overdo it so as to tear the breasts.
Layout the chicken breasts and pat them dry with a towel. Season both sides liberally with salt and pepper.
Prepare the other ingredients. Slice the 8 oz ball of mozzarella into thin, wide slices. If opting for shredded, skip this step.
Slice about 1/3 of the cherry tomatoes into thin slices, about the same thickness as the mozzarella. If using marinara sauce, don't thinly slice any of the tomatoes.
For each thin, pounded out chicken breast, spread 1- 1 1/2 tbsp pesto. Top with the mozzarella you opted for (equally divide the shredded or the sliced ball) and the tomatoes (sliced tomatoes or 1 tbsp marinara sauce).
Roll the chicken breast lengthwise tightly and secure with a toothpick. Repeat the same process with every chicken breast.
Heat a large cast-iron pan and add 1 tbsp olive oil. Starting with 6 of the rolled breasts, sear them for 4-5 minutes per side (2 sides) without moving them. Repeat with the other 6, adding olive oil as necessary. When done, turn off the heat underneath.
While the chicken breasts are searing, cut the remaining cherry tomatoes into quarters.
Place all of the chicken breasts (seared) back into the pan, squishing them a little if necessary. Top each breast with some of the remaining pesto, dividing it equally. Top with the diced cherry tomatoes, 3/4 cup shredded mozzarella, and parmesan.
Sprinkle with some pepper, then place the cast-iron pan into the preheated oven and bake for 13-15 minutes. Optionally, broil for 2-3 minutes at the end to crisp up the cheese. Enjoy!