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Remove lamb from bag, reserving marinade, then transfer to a roasting pan just large enough to hold it. Pat lamb dry and rub with oil, then season generously with salt and pepper.
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Preheat oven to 450°F. Put lamb in upper third of oven and reduce heat to 350°F. Roast until a thermometer inserted in thickest part of leg (without touching bone) registers 125°F, about 1 to 1 1/2 hours.
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Transfer lamb to a platter, cover with foil, and let stand 15 to 25 minutes (internal temperature will rise to about 135°F).
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While lamb is standing, pour reserved marinade into roasting pan. Straddle pan across 2 burners and boil marinade, scraping up brown bits, until reduced to about 1 cup.
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Add any meat juices that have accumulated on platter and whisk in jelly and salt and pepper to taste. Add butter and swirl or shake pan until incorporated. Pour sauce through a fine sieve into a sauceboat to serve with lamb.