Preheat your oven to 400°F (200°C) and lightly grease a 9×13 inch baking dish with butter or cooking spray. Set aside.
Slice the chicken breasts in half horizontally to create four cutlets. Use a rolling pin to pound the thicker ends until they are of even thickness.
In a large bowl, whisk together 1 tablespoon of olive oil, minced garlic, Italian seasoning, salt, and pepper. Add the chicken to the bowl, tossing to coat evenly. Cover and chill in the fridge for 15-30 minutes.
Heat the remaining 1 tablespoon of olive oil in a large skillet over medium heat. Add the fresh spinach and cook, stirring frequently, until wilted (about 2-3 minutes). Set aside.
In a medium bowl, beat the softened cream cheese until smooth. Add the wilted spinach, 1/2 cup of mozzarella cheese, and Parmesan cheese (if using). Mix until well combined.
Remove the chicken from the marinade, allowing any excess to drip off. If desired, sear the chicken in the skillet for 1-2 minutes per side.
Arrange the chicken in a single layer in the prepared baking dish.
Spread the spinach and cream cheese mixture over the chicken, then sprinkle the remaining 1/2 cup of mozzarella cheese over the top.
Bake for 20-30 minutes, until the chicken is cooked through and the cheese is melted and lightly browned.
Let the casserole stand for 5 minutes before serving. Enjoy!