In a large mixing bowl, combine the cooked rice, roast beef, beef broth, shredded cheddar cheese, frozen peas, garlic powder, and onion powder. Mix well.
Season with salt and pepper to taste.
Transfer the mixture to a baking dish and spread it out evenly.
Bake in the preheated oven for 45 minutes, or until the top is golden and bubbly.
Let it cool for a few minutes before serving.
Notes:
This casserole can be made ahead and refrigerated. Just bake it when you're ready to eat.