Preheat Oven: Preheat your oven to 375°F (190°C). Lightly grease a baking dish or line it with parchment paper.
Prepare the Chicken: Pat the chicken breasts dry with paper towels. Use a sharp knife to carefully make a horizontal slit along the side of each chicken breast to create a pocket. Be careful not to cut all the way through.
Season the Chicken: In a small bowl, mix together the garlic powder, onion powder, dried oregano, salt, and black pepper. Rub this seasoning mixture all over the chicken breasts, including inside the pockets.
Prepare the Filling: In a medium bowl, combine the chopped spinach, crumbled feta cheese, shredded mozzarella cheese, chopped sun-dried tomatoes (if using), minced garlic, and lemon juice. Mix until well combined.
Stuff the Chicken: Stuff each chicken breast with the spinach and feta mixture, pressing the filling inside the pockets. Secure the openings with toothpicks if necessary.
Bake the Chicken: Place the stuffed chicken breasts in the prepared baking dish. Drizzle with olive oil. Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (74°C).
Serve: Remove the toothpicks before serving. Serve the spinach and feta baked chicken hot, garnished with additional lemon slices or fresh herbs if desired.