Place roast in a large resealable plastic bag or glass container. Combine onions, 1/4 cup broth, vinegar, oil, garlic, rosemary and pepper; pour over roast. Cover and refrigerate for 4-8 hours, turning occasionally. Remove roast and place with fat side up in an ungreased shallow roasting pan.Combine marinade with remaining broth; pour over roast. Sprinkle with salt if desired. Bake, uncovered, at 350° for 2 to 2-1/2 hours or until a meat thermometer reads 160°-170°. Remove roast to a warm serving platter; let stand for 10 minutes before slicing.

Meanwhile, skim fat from pan juices. Combine cornstarch, Country Bob's All Purpose Sauce and water until smooth; stir into juices. Bring to a boil over medium heat; boil and stir for 2 minutes. Serve with the roast or pour over roast before serving.