Start by preheating your oven to 375°F (190°C). In a skillet over medium heat, cook the ground beef along with the diced onion until the beef is browned and the onion is softened. Make sure to drain any excess fat from the skillet. Next, add in the garlic powder and tomato sauce, then stir in the cooked rice and sour cream. Season the mixture with salt and pepper to your liking. Stir everything well until all ingredients are nicely combined. Pour this delicious mix into a greased baking dish and sprinkle the shredded cheddar cheese evenly on top. Pop it in the oven for about 20 to 25 minutes, until the cheese is beautifully melted and bubbly. Once done, take it out and give it a sprinkle of fresh parsley before serving it hot. Enjoy!