Heat a large skillet over medium-high heat. Add the ground beef and onions. Cook, stirring frequently, until the beef is no longer pink. Drain off any excess grease. Add the beef and onions to the crock pot. Stir in the taco seasoning, taco sauce, and refried beans. Mix just ot combine. Cover the crock pot and cook on low heat for 3-4 hours or on warm for up to 6 hours. Serve the taco mixture in taco shells and add any desired toppings such as cheese, salsa, sour cream, guacamole, lettuce, fresh onion, fresh tomato, etc.