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1
Preheat oven to 450F.
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2
With a vegetable peeler remove zest from lemon and reserve lemon.
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3
Pit olives.
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4
In a food processor finely chop zest, olives, garlic, and parsley with 2 tablespoons oil.
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5
Peel potatoes and cut into 1 1/2-inch pieces.
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6
In a large flameproof roasting pan toss potatoes with remaining 2 tablespoons oil to coat and season with salt and pepper.
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7
Arrange lamb on potatoes and with the tip of a sharp small knife cut small slits all over lamb.
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8
Rub olive mixture over lamb, pushing it into slits.
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9
Halve reserved lemon and squeeze juice over lamb.
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10
Season lamb with salt and pepper and roast with potatoes in middle of oven 20 minutes.
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11
Reduce temperature to 350F.
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12
Roast lamb and potatoes, loosening potatoes from pan with a metal spatula and turning them occasionally, 1 hour more, or until a meat thermometer inserted into thickest part of meat registers 135F.
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13
Transfer lamb to a cutting board and let stand while making gravy.
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14
Transfer potatoes to a large bowl and keep warm.
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15
In a cup with your fingers blend together flour and butter.
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16
Add wine and water to roasting pan and deglaze pan over moderately high heat, stirring and scraping up brown bits.
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17
Transfer mixture to a small saucepan and bring to a boil.
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18
Whisk in flour mixture, whisking until incorporated, and simmer gravy, stirring occasionally, 1 minute.
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19
With a slotted spoon remove any potato pieces from gravy.
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20
Garnish lamb with lemon halves and olive branches and serve with gravy and potatoes.
