Preheat the oven to 375°F and lightly grease a 9x13-inch baking dish.
Cook the ground chicken and onion in a skillet over medium heat for 6–8 minutes, breaking the chicken apart until no longer pink.
Season with garlic powder, salt, and pepper and stir well.
Add cream of chicken soup, sour cream, and chicken broth, then stir until creamy.
Mix in the frozen vegetables and cook for 2–3 minutes.
Transfer the mixture to the baking dish and spread it evenly.
Sprinkle cheddar cheese over the top and add breadcrumbs if using.
Bake uncovered for 20–25 minutes until hot and bubbling.
Let the casserole rest for about 5 minutes before serving.