Preheat your oven to 350°F (175°C).
If using leftover or rotisserie chicken, shred it into bite-sized pieces and set aside.
In a large mixing bowl, combine the condensed cream of chicken soup, sour cream, dry onion soup mix, garlic powder, salt, and pepper. Stir until well combined.
Fold in the shredded chicken and broccoli florets into the mixture.
Transfer the mixture into a greased 9x13 inch baking dish and spread it out evenly.
Sprinkle the shredded cheddar cheese evenly over the top, then add the crispy French fried onions.
Place the casserole in the preheated oven and bake for about 25-30 minutes, or until the cheese is melted and bubbly.
Remove the casserole from the oven and let it cool for a few minutes before serving.