Preheat your oven to 375°F (190°C).
In a mixing bowl, combine the chopped spinach, cream cheese, mozzarella cheese, Parmesan cheese, minced garlic, Italian seasoning, salt, and pepper. Mix until well combined.
Place the chicken breasts on a clean cutting board. Use a meat mallet or rolling pin to pound the chicken breasts to an even thickness (about ½ inch thick).
Spoon the cheese and spinach mixture evenly onto the center of each chicken breast. Be careful not to overfill.
Fold the chicken breasts over the filling, and secure them with toothpicks or tie with kitchen twine to keep the filling inside.
In a skillet, heat the olive oil over medium heat. Sear the stuffed chicken breasts for about 3-4 minutes on each side or until they are golden brown.
Transfer the seared chicken breasts to a baking dish and bake in the preheated oven for 20-25 minutes or until the chicken reaches an internal temperature of 165°F (75°C).
Remove the chicken from the oven and let it rest for 5 minutes before removing the toothpicks or twine.
Slice and serve warm, garnished with fresh herbs if desired.