01 - Preheat oven to 400°F. Lightly grease a baking dish with cooking spray or oil.
02 - Using a sharp knife, cut a horizontal pocket into the side of each chicken breast without cutting all the way through, creating a pouch for the filling.
03 - In a medium bowl, combine chopped spinach, crumbled feta cheese, minced garlic, fresh parsley, fresh dill, lemon zest, and ground black pepper. Mix until well combined.
04 - Divide the spinach-feta mixture evenly among the four chicken breasts, spooning filling into each pocket. Secure pockets with toothpicks if necessary.
05 - Rub the outside surface of each stuffed chicken breast with 1 tablespoon olive oil, then season with salt and freshly ground black pepper.
06 - Place stuffed chicken breasts in the prepared baking dish. Drizzle with remaining olive oil and sprinkle dried oregano evenly over the top.
07 - Bake for 25 to 30 minutes, or until chicken reaches an internal temperature of 165°F when measured with a meat thermometer in the thickest part, and the top is lightly golden.
08 - Remove from oven and carefully remove toothpicks. Allow to rest for 2 minutes before serving.