1 (5-7 pound) bone-in whole American leg of lamb, aitchbone removed by butcher
4 cloves garlic, minced
2 tablespoons anchovy paste
1 teaspoon Dijon mustard
½ cup unsalted butter, at room temperature
Freshly ground black pepper, as needed
Kosher salt, as needed
2 pounds small Yukon gold potatoes, halved
2 tablespoons extra-virgin olive oil
2 springs fresh rosemary
3 lemons, halved
To prepare the leg of lamb start by removing the fell (a tough layer of fat) if necessary, with a sharp knife. Next trim the fat cap so that it is about ¼” thick. Make ½” deep incisions with a sharp knife all over the meat so the rub can penetrate.
In a small mixing bowl stir together the garlic, anchovy paste, Dijon mustard and butter. Season the lamb all over with salt and pepper and rub the paste all over the lamb.
Heat oven to 450ºF. Position lamb on a rack set in a roasting pan so that the fat-cap side is down. Roast until the lamb is browned, about 15 minutes. Rotate the lamb so it’s fat-side up and lower oven heat to 350ºF. Roast lamb for 45 minutes. In a medium mixing bowl toss together the potatoes with the olive oil and rosemary and arrange under the rack holding the lamb. Continue roasting until the internal temperature of the lamb reaches 130ºF, about an additional 45 minutes to hour.
Remove the lamb from the pan to a large cutting board and let rest, tented with aluminum foil for 15 minutes.
Heat a grill pan over medium-high heat, add lemons cut side down and cook until grill marks appear, about 3 minutes. Remove and set aside.
Carve lamb against the grain into ½-inch slices and serve on a platter with potatoes and lemons arranged around the board.