Brown ground beef with onions in large skillet, then drain. Stir in next 3 ingredients. Bring to boil and simmer on low heat for 10 minutes, stirring occasionally. Meanwhile, cook franks as directed on package. Place franks in buns and top with ground beef mixture. Top with additional chopped onions and yellow mustard, if desired. To prepare on the grill, heat grill to medium. Grill franks for 7-9 minutes, or until heated through, turning occasionally.