Fry bacon, saute onions. Add in separate mixing bowl 2 teaspoons cornstarch, 1/4 teaspoon salt and 1/4 teaspoon dry mustard. Cook green beans, save 1/2 cup liquid. Add 1 tablespoon brown sugar, 1 tablespoon vinegar all but 1 tablespoon bacon drippings. Mix remaining bacon drippings with cornstarch mixture, add gradually to green beans. Serve hot.