Preheat your oven to 375°F (190°C).
Prepare the filling by combining spinach, cream cheese, mozzarella, sun-dried tomatoes, garlic, and oregano in a bowl. Mix well until evenly combined.
Take each chicken breast and make a pocket by slicing horizontally through the thickest part, being careful not to cut all the way through.
Stuff each chicken breast with about 1/4 of the filling, then secure the openings with toothpicks or tie with kitchen twine.
Season each stuffed chicken breast generously with salt and pepper on both sides.
In a skillet, heat olive oil over medium-high heat. Sear each stuffed chicken breast for 3-4 minutes on each side until golden brown.
Transfer the seared chicken to a baking dish and bake in the preheated oven for 20-25 minutes, or until the internal temperature reaches 165°F (75°C).
Once cooked, remove the chicken from the oven, and let it rest for 5 minutes before serving.
Serve sliced or whole, drizzled with any pan juices, and enjoy your flavorful creation!