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1
Heat oven to 425 degrees.
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2
Use a small sharp knife to make about a dozen incisions, each about 2 inches deep, through the fat that covers the top of the meat.
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3
Using a mortar and pestle or a blender, blend 2/3 of the anchovies (or 2/3 of the mustard if using), the rosemary leaves and the garlic cloves into a chunky paste.
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4
Using your fingers, press paste deeply into incisions.
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5
Mix remaining anchovies (or mustard) and the butter into a paste.
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6
Smear this mixture all over the surface of the roast.
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7
Season liberally with black pepper.
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8
(Do not add salt; the anchovies are salty enough, and so is the mustard.)
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9
Place the lamb on a rack in a roasting pan, fat side up, and squeeze the lemon halves over.
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10
Pour the wine around the roast into the pan.
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11
Roast 15 minutes, then reduce heat to 350 degrees and roast until internal temperature reaches 130 to 135 degrees (for medium-rare or medium meat), about another 60 to 90 minutes.
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12
Baste every 20 minutes or so with the wine and drippings in the pan, adding more wine as needed to keep the liquid from scorching.
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13
If possible, for the last 15 minutes of cooking, use convection or a broiler to crisp the fat on the roast.
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14
Remove pan from the oven, remove rack from the pan, and let the roast rest on the rack for at least 15 to 20 minutes in a warm place, tented with foil.
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15
The internal temperature will rise to about 140 to 145 degrees.
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16
To make sauce from the pan drippings, remove a few tablespoons of fat by tipping the pan and spooning off the top layer.
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17
Put the pan over medium heat until the liquid simmers.
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18
Taste the simmering liquid and whisk in more wine, 1/4 cup at a time, until the consistency and flavor are right.
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19
Do not let the mixture become syrupy; it should be a sharp jus, not a thick gravy.
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20
Carve lamb into 1/2-inch-thick slices and arrange on a heated platter, decorated with rosemary sprigs.
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21
Serve with piping hot gravy.
