Preheat your oven to 400°F (200°C) and lightly grease a 9x13-inch baking dish with cooking spray or butter.

Cut each chicken breast horizontally to create four thin cutlets. Use a meat mallet to gently pound any thicker ends to ensure even thickness throughout.

In a large bowl, whisk together 1 tablespoon of olive oil, minced garlic, Italian seasoning, salt, and black pepper. Add the chicken cutlets and toss to coat. Cover and refrigerate for 15-30 minutes to marinate.

Heat the remaining 1 tablespoon of olive oil in a medium skillet over medium heat. Add the fresh spinach and cook for 2-3 minutes, stirring occasionally, until completely wilted. Remove from heat and set aside to cool slightly.

In a medium bowl, beat the softened cream cheese until smooth and creamy. Stir in the cooked spinach, 1/2 cup of the mozzarella cheese, and Parmesan cheese if using. Mix until well combined.

Optional step: For added flavor and color, heat a skillet over medium-high heat and sear the marinated chicken cutlets for 1-2 minutes per side until lightly browned. This step can be skipped to save time.

Arrange the chicken cutlets in the prepared baking dish in a single layer. Spread the cream cheese and spinach mixture evenly over each piece of chicken. Sprinkle the remaining 1/2 cup mozzarella cheese over the top.

Bake in the preheated oven for 20-30 minutes, until the chicken reaches an internal temperature of 165°F (75°C) on a meat thermometer and the cheese is melted and lightly browned on top.

Remove from oven and let the casserole rest for 5 minutes before serving. This allows the juices to redistribute and the filling to set slightly for easier serving.