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. Add feta, cream cheese, garlic, parsley, oregano, and nutmeg. Mix until well combined.
03 - 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 with salt and pepper.
04 - 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 during baking.
05 - Place stuffed breasts in the prepared baking dish, arranging them in a single layer without overcrowding. Drizzle with the remaining tablespoon of olive oil.
06 - Bake for 25–30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F. The juices should run clear when pierced, and the top should be golden brown.
07 - Remove from oven and let rest for 5 minutes. Remove toothpicks before serving. Serve immediately while hot.