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Preheat the oven to fan 180C/ conventional 200C/gas 6. Using a small knife, trim away any excess fat from the lamb and place the fat in the roasting tin. Then criss-cross the top and sides of the lamb with the knife, making shallow incisions. Mix half the olive oil with the rosemary, salt and pepper and set aside.
Place the roasting tin over a medium heat and warm the lamb fat with the remaining olive oil for a few seconds. Add the lamb and rub the rosemary oil over the top and into the slashes. Keep turning the lamb over in the hot oil until it begins to colour all over.
Tip the vegetables and sprigs of herbs into the tin, sprinkle the lamb with a little extra salt and roast in the oven for 50 minutes or until the outside fat starts to brown. Remove the tin from the oven and turn the temperature down to fan 140C/ conventional 160C/gas 4.
Scrape the vegetables away from the bottom of the tin and turn the lamb over. Baste with a little of the fat, then continue to roast for 45-50 minutes. Test by inserting a metal skewer into the thickest part of the leg: for medium-rare meat, the tip should feel warm to the touch and the juices that run out of the meat should be fairly pink.
Remove the lamb to a warmed serving dish and cover with a loose tent of foil. This will prevent heat escaping and will allow the lamb to relax while the gravy is made.
Blanch the orange zest in boiling water for 1 minute until soft, then drain and set aside. Remove the vegetables from the roasting tin (reserve any that aren’t too dark for tucking round the roast), then put the tin on a medium heat and skim away all but 1 tbsp of the fat. Sprinkle the flour in and stir constantly. Add the wine, orange juice and zest and stock and boil rapidly for about 10 minutes until the gravy thickens, colours and is reduced by about half. Taste and add salt and pepper if necessary. Finally, stir in the redcurrant jelly and simmer until melted, then strain. Serve the lamb surrounded by any reserved vegetables, with the hot gravy.