In a medium bowl, combine the chopped spinach, feta cheese, cream cheese, garlic powder, onion powder, salt, and pepper. Mix until well combined.
Using a sharp knife, create a pocket in each chicken breast by slicing horizontally, being careful not to cut all the way through.
Stuff each chicken breast with the spinach and feta mixture, securing the opening with toothpicks if necessary.
In a large oven-safe skillet, heat the olive oil over medium-high heat. Add the stuffed chicken breasts and sear for about 3-4 minutes on each side until golden brown.
Sprinkle the dried oregano and drizzle the lemon juice over the chicken.
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 (74°C).
Remove from the oven, let rest for 5 minutes, then remove toothpicks before serving.
Notes
For added flavor, consider mixing in some sun-dried tomatoes or olives into the stuffing mixture.
Serve the stuffed chicken with a side of quinoa or a fresh salad for a complete meal.