Preheat your oven to 375°F (190°C).
In a medium bowl, combine the chopped spinach, crumbled feta cheese, cream cheese, minced garlic, dried oregano, salt, pepper, and the zest and juice of half the lemon. Mix until well combined.
Using a sharp knife, carefully create a pocket in each chicken breast by slicing horizontally, making sure not to cut all the way through.
Stuff each chicken breast with the spinach and feta mixture, dividing it evenly among the breasts. Secure the openings with toothpicks if necessary.
In a large oven-safe skillet, heat olive oil over medium-high heat. Add the stuffed chicken breasts and sear for about 3-4 minutes on each side, until golden brown.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Remove the skillet from the oven and let the chicken rest for 5 minutes before serving. Garnish with additional lemon zest and fresh herbs if desired.