Preheat & prep.
Set your oven to 350°F (175°C). Grease a 9×13 casserole dish with butter or nonstick spray.
Mix the creamy base.
In a large bowl, combine cream of chicken soup, sour cream, half the cheddar, half the mozzarella, onion, garlic powder, salt, and pepper. Stir until smooth.
Add the chicken.
Fold in shredded chicken until evenly coated with the creamy mixture.
Assemble the casserole.
Pour the mixture into the prepared dish. Smooth it out, then top with remaining cheese.
Add crunch.
Sprinkle French fried onions or breadcrumbs on top for that golden, crispy finish.
Bake to perfection.
Bake 25–30 minutes until bubbly and the topping is golden brown.
Cool slightly and serve.
Let it rest for 5 minutes so it sets a bit. Then dig in!