Instructions: Place roast in a large resealable plastic bag or container. Combine onions, 1/4 cup broth, vinegar, oil, garlic, rosemary, and pepper; pour over roast. Cover and refrigerate for 4 to 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 degrees for 2 to 2 1/2 hours or until a meat thermometer reads 160-170 degrees. Remove roast to a warm serving platter; let stand for 10 minutes before slicing.

Meanwhile, skim fat from pan juices. Combine cornstarch and water until smooth; stir in juices. Bring to a boil over medium heat; boil and stir for 2 minutes. Serve with the roast.

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