Preheat oven to 350°F. Stir together onion, oil, Worcestershire, dry mustard and pepper. Season to taste with salt. Place roast on rack in shallow roasting pan. Insert meat thermometer in thickest part of roast.
Roast 1 1/2 to 1 3/4 hours or until thermometer registers 140°F for medium-rare. Remove roast from pan; cover with foil and let stand 15 minutes* before carving. Temperature will rise to 145°F during stand time.
To make au jus, remove most of the fat from drippings from pan. Add 2 cups boiling water to pan; stir to scrape up browned bits from bottom of pan. Bring to boiling; simmer 2 minutes more. Season to taste with salt and pepper. Serve with sliced roast.
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