Brown meat in large heavy skillet over moderate heat for 2 to 3 minutes, breaking up clumps. Add onion, mustard, worcestershire, sugar, vinegar, ketchup, salt, green pepper and celery. Reduce heat to low and cook, stirring now and then, about 40 minutes until flavors mingle. Place 1/2 hamburger bun on each of four plates and ladle meat sauce generously on top. Serve at once.