Preheat your oven to 375°F (190°C).
In a mixing bowl, combine spinach, cream cheese, mozzarella, sun-dried tomatoes, garlic, Italian seasoning, salt, and pepper.
Carefully butterfly each chicken breast, making a pocket for the filling.
Stuff each chicken breast with the prepared mixture, securing with toothpicks if necessary.
Heat olive oil in a large oven-safe skillet over medium-high heat.
Sear the stuffed chicken breasts for 3-4 minutes on each side or until browned.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and let the chicken rest for 5 minutes before slicing.
Serve warm, garnished with fresh herbs if desired.