01 - Preheat oven to 400°F. Lightly grease a baking dish with cooking spray or oil.
02 - In a skillet over medium heat, add chopped spinach and cook for 1–2 minutes until wilted. Transfer to a bowl and let cool slightly.
03 - Add feta, cream cheese, garlic, parsley, oregano, and nutmeg to the cooled spinach. Mix until well combined into a uniform mixture.
04 - Pat chicken breasts dry with paper towels. Using a sharp knife, cut a horizontal pocket into the side of each breast, being careful not to cut through completely. Season both sides generously with salt and pepper.
05 - Divide the spinach mixture evenly among the 4 chicken breasts, stuffing each pocket. Secure the opening with toothpicks if needed to prevent filling from escaping.
06 - Place stuffed breasts in the prepared baking dish. Drizzle with 1 tablespoon olive oil. Bake for 25–30 minutes until the chicken is cooked through (internal temperature reaches 165°F) and juices run clear.
07 - Remove toothpicks before serving. Serve immediately while hot.