Lay lamb out with the fat cap down. Trim any visible fat or connective tissue away. Turn over and trim fat cap down to 1/8 to 1/4" thickness. Pound roast to an even 1" thickness. Score fat cap in diagonal cuts, down to, but not into, meat, at 1/2" intervals, making a diamond crosshatch. Rub both sides with 2 T salt, being sure to rub salt down into scored slits. Let stand, uncovered at room temperature, for 1 hour.
Place an oven rack 4-5 inches below broiler. Preheat oven to 250°F On a rimmed baking sheet, toss together oil, sliced shallot, garlic, ginger, coriander, cumin, mustard, bay leaves, and lemon zest. Bake, stirring occasionally, until spices are soft and fragrant, and shallots and garlic are golden, about 1 hour. Remove and discard bay leaves.
Pat lamb very dry with paper towels and place on top of spices, fat cap up. Roast on lower-middle rack until it reaches 120F internal temperature, 30-40 minutes. Remove sheet from oven and switch to broiler. When broiler is hot, place sheet on upper rack and broil until fat cap is well-browned, and even charred in occasional spots. Internal temperature should now be 125F for medium rare, 3-8 minutes.
Remove sheet from oven and transfer meat (using 2 pairs of tongs) to carving board. Tent loosely with foil and let rest 20 minutes.
Meanwhile, pour pan juices through fine-mesh sieve into medium bowl, pressing solids to extract as much liquid as possible. Discard solids. Stir in mint, cilantro, minced shallot and lemon juice. Drain any juices accumulated on carving board into bowl and season with salt and pepper to taste.
To carve, cut lamb across the shorter direction (with the grain) into three equal pieces. Turn each piece 90 degrees and slice across the grain into 1/4" slices. Serve with sauce (if cooled, reheat sauce in microwave before serving).
Optional alternate flavor: omit cumin and mustard seeds. Instead, add the needles from 6 sprigs of fresh rosemary and 1/2 tsp crushed red pepper flakes. Replace cilantro with parsley in sauce.