Preheat the oven to 375°F (190°C).
In a mixing bowl, combine chopped spinach, cream cheese, mozzarella cheese, Parmesan cheese, minced garlic, dried oregano, salt, pepper, and sun-dried tomatoes, if using. Mix well until creamy and well combined.
Using a sharp knife, make a pocket in each chicken breast by slicing horizontally, making sure not to cut through completely.
Stuff each chicken breast with the spinach and cheese mixture, dividing the filling evenly among all breasts. Secure the openings with toothpicks or tie with kitchen twine if needed.
Heat olive oil in a skillet over medium-high heat. Sear the stuffed chicken breasts for about 2-3 minutes on each side, until golden brown.
Transfer the seared chicken breasts to a baking dish. Bake in the preheated oven for about 20 minutes or until the chicken is fully cooked and reaches an internal temperature of 165°F (75°C).
Remove from the oven and let rest for 5 minutes before removing toothpicks or twine. Slice and garnish with fresh herbs before serving.