Preheat your oven to 350°F (175°C). This ensures that your casserole cooks evenly and develops a golden crust on top.
If you haven't already, steam your broccoli florets until they are bright green and tender, about 4-5 minutes. Drain and set aside.
In a large mixing bowl, combine the cooked rice, shredded chicken, steamed broccoli, cream of chicken soup, milk, diced onion, garlic powder, black pepper, and salt. Stir well until all the ingredients are evenly coated and mixed.
Fold in 3/4 cup of the shredded cheddar cheese into the mixture, saving the rest for the topping.
Pour the mixture into a greased 9x13 inch baking dish, spreading it out evenly.
In a small bowl, combine the breadcrumbs with the remaining 1/4 cup of shredded cheddar cheese and drizzle with olive oil. Mix until the breadcrumbs are well coated.
Sprinkle the breadcrumb mixture evenly over the top of the casserole, creating a crunchy, cheesy layer.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and the casserole is bubbling.
Once out of the oven, let the casserole sit for about 5-10 minutes before serving.