Heat the oil in a large soup pot over medium heat; cook and stir the onion and garlic until the onion is translucent, about 5 minutes. Add the beef stew meat and quickly brown the pieces on all sides.
Stir in the pearl barley and transfer the stew to a slow cooker set on Low until the barley is tender, about 12 hours.
Stir in the potatoes, sweet potato, baked beans, tomato sauce, barbecue sauce, mustard, onion soup mix, seasoned salt and steak seasoning and pour in enough water to cover. Bring the mixture to a boil, reduce to a simmer and cook on low heat until the beef is tender, 1 1/2 to 2 hours, stirring occasionally.