Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish with cooking spray or butter.
In a large mixing bowl, combine the cooked rice, shredded cheddar cheese, milk, sour cream, green onions, garlic powder, onion powder, black pepper, salt, and frozen mixed vegetables. Stir until well combined.
Pour the rice mixture into the prepared baking dish, spreading it evenly.
If using breadcrumbs, melt the butter in a small pan over low heat. Stir in the breadcrumbs until they are coated with butter. Sprinkle the breadcrumb mixture evenly over the top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the top is golden brown.
Remove from the oven and let it cool for a few minutes before serving.