Crumble the ground beef and onion into a large skillet over medium-high heat. Cook, stirring to break up lumps, until no longer pink. Drain off the grease and return to the stove. Add the Worcestershire sauce, beef broth, salt, pepper, and butter. Bring to a boil, then set the heat to low and simmer uncovered until the liquid is almost completely gone, about 40 minutes. Remove from the heat, cover, and let rest for 15 minutes before serving on buns.