Preheat oven to 400°F (200°C). Lightly grease a 9×13-inch baking dish.

Cut chicken breasts in half horizontally to make four thin cutlets. Pound thick ends gently to even thickness.

In a bowl, whisk 1 tablespoon olive oil, garlic, Italian seasoning, salt, and black pepper. Coat chicken in mixture, cover, and refrigerate for 15–30 minutes.

In a skillet, heat remaining olive oil over medium heat. Add spinach and cook 2–3 minutes until wilted. Set aside.

In a bowl, beat softened cream cheese. Stir in spinach, ½ cup mozzarella, and Parmesan if using. Mix until combined.

(Optional) Sear chicken in a hot skillet 1–2 minutes per side for added flavor.

Place chicken in baking dish in a single layer. Spread cream cheese mixture evenly over chicken. Top with remaining mozzarella.

Bake 20–30 minutes, until chicken reaches 165°F (75°C) and cheese is melted and lightly golden.

Let casserole rest 5 minutes before serving.