Preheat your oven to 375°F.

In a medium bowl, combine the chopped spinach, cream cheese, mozzarella cheese, Parmesan cheese, minced garlic, Italian seasoning, salt, and pepper. Mix until well combined.

Using a sharp knife, carefully create a pocket in each chicken breast by slicing horizontally, but not all the way through.

Stuff each chicken breast with the spinach and cheese mixture, securing the opening with toothpicks if necessary.

Heat the olive oil in an oven-safe skillet over medium-high heat. Add the stuffed chicken breasts and sear for 3-4 minutes on each side until golden brown.

Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F.

Remove from the oven and let rest for 5 minutes before serving.