Preheat the oven to 425ºF. Prepare a baking dish or pan by spraying lightly with cooking spray.
Heat olive oil in a small skillet over medium heat. Add onion and two-thirds of the garlic and cook until softened, 3 to 4 minutes. Transfer to a bowl to cool slightly, then add the spinach and feta; season with pepper and nutmeg.
Add butter to skillet, heat until melted. Turn off the heat and add panko, remaining garlic, oregano, and salt; toss to combine.
Split each chicken breast across, but not through, and open like a book (be careful, you can easily cut yourself doing this!). Stuff with the spinach and feta mixture, dividing the stuffing evenly, and letting the stuffing overflow at the edges. Fold the top half of the chicken back over the stuffing. Place each chicken breast in the baking dish, sides not touching, and top evenly with the breadcrumbs.
Bake until the juices run clear, or until internal temperature measures 165°F with an instant read thermometer, about 45-50 minutes.
Video
Notes
Adapted from Rachael Ray Magazine.
Add Ins: Add extra garlic, chopped sun-dried tomatoes, or kalamata olives to the stuffing.
To make keto or gluten-free, omit the breading. Season chicken breasts with salt, pepper, and dried oregano. Spray lightly with olive oil spray before baking.
Make Ahead Tip: Prepare the chicken breasts, making sure the stuffing is cool before stuffing the breasts. Arrange them in a baking dish, top with the panko breading, and cover the dish tightly. Refrigerate up to 24 hours in advance. Remove from refrigerator 15 minutes in advance. Bake as directed in a preheated oven.