1Place beef in resealable food-storage plastic bag or 3-quart casserole, cutting beef in half if necessary to fit. In medium bowl, stir vinegar, chili sauce, brown sugar, basil and salt until well blended. Pour over beef; turn to coat. Seal bag or cover casserole; refrigerate, turning beef once, at least 8 hours but no longer than 24 hours to marinate.
2Heat oven to 325°F. Place beef and marinade in 4-quart Dutch oven or 3-quart casserole. Place onions over top; cover. Roast 2 hours, brushing beef twice with marinade.
3Remove cover; roast 1 hour longer, brushing beef frequently with marinade, until beef is tender. Cover and let stand 10 minutes.
4To serve, cut beef across grain into thin slices; arrange on serving platter. With slotted spoon, place onions over beef. Sprinkle with parsley. Discard cooking liquid.