We prefer the subtler flavor and larger size of lamb labeled “domestic” or “American” for this recipe. The amount of salt (2 tablespoons) in step 1 is for a 6 pound leg. If using a larger leg (7 to 8 pounds), add an additional teaspoon of salt for every pound.
1. For the Lamb: Place lamb on cutting board with fat cap facing down. Using sharp knife, trim any pockets of fat and connective tissue from underside of lamb. Flip lamb over, trim fat cap so it’s between 1/8 and 1/4 inch thick, and pound roast to even 1 inch thickness. Score fat cap in 1/2-inch crosshatch pattern, being careful to cut down to but not into meat. Rub 2 tablespoons salt over entire roast and into slits. Let stand, uncovered, at room temperature for 1 hour.
2. Meanwhile, adjust 1 oven rack to lower-middle position and second rack 4 to 5 inches from broiler element. Heat oven to 250 degrees. Stir together oil, shallots, garlic, ginger, coriander seeds, fennel seeds, peppercorns, bay leaves, and lemon zest on rimmed baking sheet and bake on lower rack until spices are softened and fragrant and shallots and garlic turn golden, about 1 hour. Remove sheet from oven and discard bay leaves.
3. Thoroughly pat lamb dry with paper towels and transfer, fat side up, to sheet (directly on top of spices). Roast on lower rack until lamb registers 120 degrees, 30 to 40 minutes. Remove sheet from oven and heat broiler. Broil lamb on upper rack until surface is well browned and charred in spots and lamb registers 125 degrees (for medium-rare), 3 to 8 minutes.
4. Remove sheet from oven and, using 2 pairs of tongs, transfer lamb to carving board (some spices will cling to bottom of roast); tent with aluminum foil and let rest for 20 minutes.
5. For the Sauce: Meanwhile, carefully pour pan juices through fine-mesh strainer into medium bowl, pressing on solids to extract as much liquid as possible; discard solids. Stir in parsley, cilantro, shallot, and lemon juice. Add any accumulated lamb juices to sauce and season with salt and pepper to taste.
6. With long side facing you, slice lamb with grain into 3 equal pieces. Turn each piece and slice across grain into 1/4 inch-thick slices. Serve with sauce. (Briefly warm sauce in microwave if it has cooled and thickened.)