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Preheat the oven to 450 degrees F. Place the oven rack so the lamb will sit in the middle of the oven.
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With a sharp knife cut slits all over the lamb (top and bottom). Insert the slivers of garlic.
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Brush the lamb with the oil and season with rosemary, salt and pepper. Allow to sit at room temperature for 30 minutes to 1 hour.
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Toss the potatoes and remaining garlic in a bowl with the olive oil and sprinkle with salt.
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Place potatoes in the bottom of a large roasting pan. Nestle the heads of garlic and rosemary sprigs and amongst the potatoes.
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Place the lamb on top of the potatoes and roast for about 2 hours, basting often with the pan juices. The lamb is done when it is so tender that a fork just slides through it like softened butter.
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Remove from the oven and put the lamb on a platter; cover tightly with aluminum foil. Allow the lamb to rest for about 20 minutes.
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Slice with the potatoes, giving each person a slice of lamb and a whole head of garlic, moistened with some of the pan juices. To eat the garlic, squeeze the cloves with the flat of a knife and the softened flesh will ooze out. Either smear onto the lamp or scoop up with good crusty bread.