Preheat the oven: Set your oven to 400°F (200°C) and lightly grease a baking dish.
Prepare the filling: In a medium bowl, mix together the spinach, artichoke hearts, cream cheese, mozzarella, Parmesan, and garlic until well combined.
Slice chicken pockets: Using a sharp knife, cut a pocket into the side of each chicken breast—careful not to cut all the way through.
Stuff the chicken: Spoon the spinach artichoke mixture into each pocket. Use toothpicks to hold them closed if needed.
Season the outside: Rub each breast with olive oil and sprinkle with salt and pepper.
Bake the chicken: Place the stuffed breasts in the baking dish and bake for 25 minutes, or until the chicken reaches an internal temperature of 165°F.
Rest and garnish: Let the chicken rest for 5 minutes after baking. Sprinkle with chopped parsley for a fresh finish.
Serve warm: Slice and serve with your favorite side dish like roasted potatoes, rice, or a crisp salad.