Preheat the oven to 400°F (200°C). Lightly grease a 9×13-inch baking dish with butter or non-stick cooking spray. Set aside.

Slice each chicken breast horizontally to create four thinner cutlets. Pound the thicker portions gently with a rolling pin to achieve even thickness throughout.

In a large bowl, whisk together 1 tablespoon olive oil, minced garlic, Italian seasoning, salt, and pepper. Add the chicken cutlets, ensuring they are evenly coated. Cover and refrigerate for 15 to 30 minutes to allow the flavors to infuse.

In a large skillet, heat the remaining 1 tablespoon of olive oil over medium heat. Add the fresh spinach and sauté, stirring frequently, until wilted, approximately 2 to 3 minutes. Remove from heat and set aside.

In a separate medium-sized bowl, beat the softened cream cheese until smooth. Incorporate the cooked spinach, 1/2 cup of shredded mozzarella, and Parmesan cheese (if using), mixing until well combined and creamy.

Remove the marinated chicken from the refrigerator and allow any excess marinade to drip off. If desired, sear the chicken in a skillet for 1 to 2 minutes per side for added flavor and color. Arrange the chicken in a single layer in the prepared baking dish.

Spread the spinach and cream cheese mixture evenly over the arranged chicken cutlets. Sprinkle the remaining 1/2 cup of shredded mozzarella cheese over the top.

Transfer the baking dish to the preheated oven and bake for 20 to 30 minutes, or until the chicken is fully cooked and the cheese has melted and developed a light golden color. Allow the casserole to rest for 5 minutes before serving.