sloppy joes    1 medium onion  1 pound ground beef  1 1/2 tsp. flour  2/3 cups ketchup  3/4 cups cola  2 Tbs. vinegar  1 Tbs. worcestershire sauce  1 tsp. honey  1 tsp. dry mustard (or 1 Tbs. prepared mustard)  Salt and pepper to taste  12 hamburger buns