Halve preserved lemon; remove and discard seeds and flesh. Chop skin and pulse with garlic, shallot, chiles, mint and marjoram leaves, lemon zest and juice, and salt in a food processor until coarsely ground. Stir in oil. Reserve 3/4 cup marinade.
Place lamb in a baking dish; rub with remaining marinade. Refrigerate, covered, overnight.
3. Let lamb stand at room temperature 1 hour. Heat grill to medium-high. Remove lamb from marinade, brushing off excess. Grill, covered, 8 minutes. Flip; continue cooking until a thermometer inserted in center reads 130 degrees for medium-rare, about 8 minutes more. Cover loosely with foil; let rest 20 minutes. Slice thinly and serve with herb sprigs, lemons, and reserved marinade.