Begin by heating a large skillet over medium heat. Add the ground beef and diced onions, cooking until the beef is browned and the onions are soft. Drain any excess grease and season with garlic powder, salt, and pepper.

In a large mixing bowl, combine the cooked beef mixture, cooked rice, and cream of mushroom soup. Stir until all ingredients are evenly coated and seasoned.

Grease a 9x13-inch casserole dish and transfer the mixture into it. Spread it out evenly, pressing lightly to pack it in. If desired, sprinkle shredded cheddar cheese on top for an irresistible golden crust.

Preheat your oven to 375°F (190°C). Bake the casserole for 20–25 minutes, or until the cheese is melted and bubbly. Let it cool slightly before serving.