01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish.
02 - Rub chicken breasts with olive oil, salt, and black pepper. Arrange in a single layer in the prepared baking dish.
03 - In a large bowl, combine spinach, artichoke hearts, cream cheese, sour cream, mayonnaise, garlic, mozzarella, Parmesan, onion powder, and red pepper flakes if using. Mix until smooth and well combined.
04 - Evenly spread the spinach-artichoke mixture over the chicken breasts.
05 - Sprinkle the topping mozzarella and Parmesan evenly over the casserole.
06 - Bake uncovered for 25-30 minutes, or until the chicken is cooked through (internal temperature of 165°F) and the top is golden and bubbly.
07 - Let rest for 5 minutes before serving. Serve with a side salad or steamed vegetables if desired.