Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, steamed broccoli, cooked rice, cheddar cheese, cream of chicken soup, milk, garlic powder, onion powder, salt, and pepper. Mix well until all ingredients are evenly coated.
Transfer the mixture into a greased 9x13 inch baking dish, spreading it out evenly.
If using breadcrumbs, mix them with olive oil in a small bowl until coated, then sprinkle evenly over the casserole.
Bake in the preheated oven for 25-30 minutes, or until the casserole is heated through and the top is golden brown.
Remove from the oven and let it cool for a few minutes before serving.