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Heat oven to 240C/fan 220C/gas 9. Pound the garlic, half the oregano, lemon zest and a pinch of salt in a pestle and mortar, then add the lemon juice and a drizzle of olive oil. Stab the lamb all over with a sharp knife, then push as much of the herb paste as you can into the holes.
Tip the potatoes into a large roasting tin, then toss in the remaining olive oil and any remaining herb paste. Nestle the lamb amongst the potatoes, roast for 20 mins, then reduce the temperature to 180C/fan 160C/gas 4. Roast for 1 hr 15 mins for medium-rare, adding another 15 mins if you prefer your lamb medium. Baste the lamb once or twice with the juices and toss the potatoes. When the lamb is done to your liking, remove from the tin and let it rest. Throw the rest of the oregano in with the potatoes, scoop from the tin and keep warm.
Place the roasting tin over a medium flame, add the canned tomatoes and olives to the pan juices, then simmer for a few mins. Serve the lamb with the potatoes and sauce and a simple salad.