Place roast in a roasting pan; add onion, and sprinkle with salt and pepper.
Bake, covered, at 325° for 3 hours and 30 minutes. Cool and thinly slice. Place in an ovenproof container; add Spicy Sauce. Cover and chill 8 hours. Remove from refrigerator, and let stand 30 minutes.
Bake at 350° for 45 minutes or until thoroughly heated. Garnish, if desired. Serve with mashed potatoes, if desired.
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