Mint salsa verde brightens up earthy roasted lamb. Remove the lamb from the fridge about 2 hours before you want to start cooking to coat with the herb salsa to infuse the flavor and bring the meat to room temperature. Recipe and accompanying video courtesy of the American Lamb Board.
With a food processor running, add the garlic cloves. When finely minced, stop the motor, remove the top, and scrape down the bowl. Add the parsley, mint, anchovies, capers, salt, and pepper. Pulse until the ingredients are very finely chopped. Pour in 2 tablespoons of the olive oil and pulse just to combine. Transfer about two-thirds of the paste to a small bowl and cover with the remaining 1/3 cup of olive oil so that it is completely submerged. (The oil keeps the herbs bright green and will be stirred in later to make a sauce for the roast.) Cover tightly and refrigerate until serving time, or up to 2 days. The remaining one-third of the paste will be used to rub the meat.
Lay the lamb flat on a cutting board fat-side up. If there is more than ⅛-inch of fat, trim the excess with a sharp knife. Turn the meat over and trim away any large chunks of fat from the interior, but leave the rest intact. Be careful not to trim away the connective tissue that holds the meat together. Sprinkle the lamb on both sides with 2 teaspoons of the salt.
Spread the reserved salsa verde paste over the interior of the meat (not the fat side) all the way to the edges, rubbing it into the crevices. Roll the lamb up lengthwise into a cylinder. Use kitchen twine to tie the cylinder at 1- to 1 ½-inch intervals to secure the roast and make it an even thickness throughout. Tie another piece of twine lengthwise to secure any flap pieces at either end. Allow the lamb to sit at room temperature for 1 to 2 hours before roasting to come to room temperature and infuse the flavor. (The lamb can be seasoned and tied up to 2 days ahead. If you plan to wait more than 2 hours before roasting, loosely cover and refrigerate the roast, but remove it to come to room temperature before cooking. Pat the outside of the roast dry using paper towels before roasting.)
Position an oven rack in the lower third of the oven (so that the roast will be positioned in the middle) and preheat to 325 degrees F.
Trim the dark green tops from the leeks. Halve the leeks lengthwise and rinse well. Pat them dry and cut into 2-inch pieces. Put the leeks and halved potatoes in a large roasting pan. Drizzle the vegetables with olive oil, season with the remaining ½ teaspoon salt, and toss to coat. Spread them around the roasting pan in an even layer. Place the lamb roast on top, seam-side down, and transfer to the oven. Roast until an instant-read thermometer inserted into the center of the meat reaches 135 degrees F for medium-rare, 1 to 1 ½ hours.
Transfer the lamb to a cutting board and allow the meat to rest for 20 to 30 minutes, loosely covered with foil. (The internal temperature will increase to 145 degrees F.) Remove the salsa verde from the refrigerator to come to room temperature. Toss the vegetables in the roasting pan to coat them in the pan drippings. If they are tender and nicely browned in spots, set them aside in a warm spot while the meat rests. If not, return them to the oven to finish cooking.
Trim away the kitchen twine and cut the roast into ½-inch thick slices. Arrange the slices over the vegetables and pour any juices from the cutting board over the meat. Stir the salsa verde to incorporate the oil, spoon it over the lamb to serve.