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1
Bring water to a boil with garlic and peppercorns.
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2
Add hibiscus flowers and gently simmer 5 minutes.
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3
Remove from heat and let marinade steep 30 minutes.
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4
Pour marinade through a fine sieve into a bowl, pressing on solids, then discard solids.
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5
Add sugar, stirring until dissolved, and chill until cold.
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6
Remove most of fat from lamb and put lamb in double layer of sealable plastic bags with marinade.
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7
Marinate lamb, chilled, turning bag over once or twice, 12 to 24 hours.
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8
Remove lamb from bag, reserving marinade, then transfer to a roasting pan just large enough to hold it.
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9
Pat lamb dry and rub with oil, then season generously with salt and pepper.
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10
Preheat oven to 450F.
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11
Put lamb in upper third of oven and reduce heat to 350F.
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12
Roast until a thermometer inserted in thickest part of leg (without touching bone) registers 125F, about 1 to 1 1/2 hours.
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13
Transfer lamb to a platter, cover with foil, and let stand 15 to 25 minutes (internal temperature will rise to about 135F).
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14
While lamb is standing, pour reserved marinade into roasting pan.
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15
Straddle pan across 2 burners and boil marinade, scraping up brown bits, until reduced to about 1 cup.
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16
Add any meat juices that have accumulated on platter and whisk in jelly and salt and pepper to taste.
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17
Add butter and swirl or shake pan until incorporated.
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18
Pour sauce through a fine sieve into a sauceboat to serve with lamb.
