Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, brown the ground beef and diced onion until the beef is fully cooked and the onion is translucent. Drain any excess fat.
Stir in the uncooked rice, beef broth, cream of mushroom soup, garlic powder, paprika, salt, and pepper. If using, add the frozen mixed vegetables. Mix well.
Transfer the mixture to a greased 9x13-inch baking dish.
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil, sprinkle the shredded cheddar cheese on top, and return to the oven. Bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving.