In a large skillet over medium heat, cook the ground beef and onion until the beef is browned. Drain any excess fat.
Transfer the beef mixture to the slow cooker. Add the mushrooms, beef broth, garlic powder, salt, and pepper. Stir to combine.
Cover and cook on low for 6 hours.
Stir in the egg noodles, cover again, and cook for 30 more minutes until the noodles are tender.
Turn off the heat. Stir in the sour cream and 1 1/2 cups of the cheddar cheese until the sauce is smooth and the cheese has melted.
Sprinkle the remaining 1/2 cup of cheese over the top and let it melt before serving.