01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or a small amount of olive oil.
02 - Rub chicken breasts evenly with olive oil, then season both sides with salt and black pepper. Arrange in a single layer in the prepared baking dish.
03 - In a large mixing bowl, combine spinach, artichoke hearts, cream cheese, sour cream, mayonnaise, minced garlic, 1 cup mozzarella, ½ cup Parmesan, onion powder, and red pepper flakes if using. Mix thoroughly until smooth and well incorporated.
04 - Evenly distribute the spinach-artichoke mixture over each chicken breast, covering completely.
05 - Sprinkle the remaining ½ cup mozzarella and 2 tablespoons Parmesan cheese evenly over the top of the casserole.
06 - Bake uncovered for 25-30 minutes until chicken reaches an internal temperature of 165°F and the cheese topping is golden brown and bubbly.
07 - Let the casserole rest for 5 minutes before serving to allow juices to redistribute. Serve alongside a crisp green salad or steamed vegetables.