All American Barbecue Sandwiches (for a gang)

Source: Foodgeeks

4-1/2 lb. ground beef
1-1/2 cups onion, chopped
2-1/4 cups catsup
3 tbsp. prepared mustard
3 tbsp. Worcestershire sauce
2 tbsp. vinegar
2 tbsp. sugar
1 tbsp. salt
1 tbsp. pepper
18 hamburger buns, split

In a Dutch oven, cook beef and onion until meat is browned and onion is tender; drain. Combine catsup, mustard, Worcestershire, vinegar, sugar, salt and pepper; stir into beef mixture. Heat through. Serve on buns.