Preheat your oven to 375°F (190°C) to ensure even cooking.
Using a sharp knife, carefully slice each chicken breast horizontally to create a pocket.
Mix together the chili powder, Italian seasoning, black pepper, and salt. Rub this seasoning mixture inside and outside of the chicken breasts.
In a mixing bowl, combine the spinach, cream cheese, Parmesan cheese, mozzarella cheese, minced garlic, and additional pepper. Mix until well combined.
Spoon the filling into each pocket of the chicken breasts. Secure with toothpicks if necessary.
Place the stuffed chicken breasts in a baking dish and bake for about 25-30 minutes, or until cooked through.
Let the chicken rest for a few minutes before slicing. Serve with your favorite sides!