Serves 6

3½ to 4½ pound butterflied leg of lamb

1 cup walnuts

1 tablespoon canola oil

1 medium onion, chopped

1 tablespoon sherry vinegar

2 tablespoons medium dry sherry

2 cups tightly packed parsley leaves, washed

3 cloves garlic, thickly sliced

2 tablespoons Dijon mustard

2 sprigs rosemary, leaves removed

3 tablespoons olive oil

½ teaspoon salt

¼ teaspoon black pepper

Grated rind of 1 orange

¼ cup grated Parmesan

Extra olive oil (for sprinkling)

1. Remove the lamb from the refrigerator about 30 minutes before cooking.

2. Set the oven at 400 degrees. Have on hand a roasting pan fitted with a rack.

3. In a large skillet, toast the walnuts over medium-low heat for 8 minutes, shaking the pan often, or until they are aromatic. Remove the nuts from the pan; cool.

4. Wipe out the skillet. Set it over medium heat and add the canola oil. When it is hot, add the onion and cook, stirring often, for 15 minutes, or until the onion softens and turns golden. Add the sherry vinegar and sherry and bring to a boil. Cook, stirring, until most of the liquid has evaporated; cool.

5. In a food processor, combine the parsley, garlic, mustard, rosemary leaves, olive oil, salt, and pepper. Pulse the mixture until the leaves are coarsely chopped. Add the onion, orange rind, walnuts, and Parmesan, and pulse until the walnuts are coarsely chopped. Taste for seasoning and add more salt and pepper, if you like. Transfer half the mixture to a bowl; cover and refrigerate.

6. Trim excess fat from the lamb. Lay the lamb, fat side down, on a cutting board (the side that was cut from the bone should be facing up). Sprinkle with salt and pepper. Use a rubber spatula to spread the remaining pesto mixture on the meat, leaving a 1-inch border.

7. Carefully, roll the lamb up, making sure the stuffing doesn’t ooze out. Roll the meat so the seam side faces down. Tie with kitchen string at 2-inch intervals. If some of the pesto spills out, use your fingers to stuff it back into the ends of the lamb.

8. Rub the lamb lightly with olive oil and sprinkle with salt and pepper. Place the lamb on the rack in the pan. Roast for 20 minutes per pound (about 1 hour and 15 minutes) or until a meat thermometer inserted into the thickest part of the meat registers 130 degrees for medium rare. Check the lamb temperature after the meat has cooked for 50 minutes. When done, cover loosely with foil and rest for 15 minutes before carving.

9. Cut the lamb into ½-inch slices. As you cut, use your other hand to hold the slice intact to prevent the pesto from spilling out. Arrange on a platter and serve with the remaining pesto and more grated Parmesan on the side. Gordon Hamersley

Gordon Hamersley can be reached at cookingwithgordonhamersley@gmail.com