In a mixing bowl, combine the chopped fresh spinach (or thawed frozen spinach), cream cheese, Parmesan cheese, mozzarella cheese, minced garlic, and pepper. Mix until well combined.
Take your chicken breasts and place them on a cutting board. Use a meat tenderizer or rolling pin to gently pound them to an even thickness, about 1/2 inch thick. Season both sides of the chicken with chili powder, Italian seasoning, black pepper, and salt.
Divide the spinach and cheese mixture among the chicken breasts, placing it in the center of each. Carefully fold the chicken over the filling, tucking the edges in if necessary. Secure the chicken with toothpicks if desired.
Preheat your oven to 375°F (190°C). Place the stuffed chicken breasts in a lightly greased baking dish. Bake for about 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Once out of the oven, let the chicken rest for a few minutes before slicing. Serve it warm with a side of your favorite low-carb veggies or salad.