In the same skillet over medium heat, brown the onions for 5 minutes in the fat left over from the patties. Deglaze with the vinegar and let reduce until almost dry. While whisking, add the broth, flour, Worcestershire sauce and the remaining pepper. Bring to a boil and simmer over medium heat until the sauce thickens, stirring occasionally. Remove from the heat and whisk in the mustard. Return the meat to the skillet and cook for another 2 minutes or until the patties are cooked through. Adjust the seasoning.