Preheat oven to 375°F (190°C).
Cut a deep pocket into the side of each chicken breast, being careful not to cut all the way through. Season the outside with half of the salt, pepper, and garlic powder.
In a medium bowl, combine chopped spinach, sun-dried tomatoes, feta cheese, oregano, and the remaining salt, pepper, and garlic powder.
Stuff each chicken breast pocket evenly with the spinach and feta mixture. Secure the openings with toothpicks if necessary.
Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the stuffed chicken for 4-5 minutes per side until golden brown.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven, cover loosely with foil, and let rest for 5 minutes before serving.