One whole bone-in lamb shoulder, about 5 to 6 pounds
1½ tablespoons whole coriander seeds
1 tablespoon whole cumin seeds
2 teaspoons sweet paprika
½ teaspoon sugar
6 cloves garlic, smashed and peeled
8 tablespoons (1 stick) unsalted butter, softened
For serving (optional): 2 tablespoons whole cumin seeds and 4 tablespoons fleur de sel
1. Trim the lamb: Using a thin-bladed sharp knife, trim any leathery membrane and some of the excess fat from the exterior of the lamb; leave a thin (at least 1⁄8-inch) layer of fat to protect the meat from drying out.
2. Make the spice rub: In a small, dry skillet set over medium-low heat, combine the coriander and 1 tablespoon cumin and toast, shaking the pan a few times, until fragrant and beginning to darken, about 2 to 3 minutes. Let the spices cool slightly, then grind them to a coarse powder in a mortar or spice grinder. Transfer the powder to a small bowl and stir in the paprika and sugar. Combine the garlic and 2 teaspoons of kosher salt in the mortar and pound to a paste (alternatively, you can make the garlic paste on a cutting board using a chef's knife). Add the garlic paste to the spices. Using a wooden spoon, work in the butter until evenly mixed.
3. Season the lamb: Using the tip of a paring knife, make a dozen or more 1-inch-deep incisions in the meatiest part of the lamb. Rub the lamb all over with the seasoned butter, doing your best to smear some into the incisions. Set the lamb in a large baking dish and refrigerate, uncovered or loosely covered, for 1 to 2 days. Let the lamb sit at room temperature for 1 to 2 hours before roasting.
4. Preheat the oven to 450°. Position a rack in the lower third of the oven and heat to 450° (425° in a convection oven).
5. Place the lamb in a 13-by-9-inch roasting pan, skin side up. Roast for 25 minutes, then lower the oven temperature to 325° and continue roasting, basting occasionally with any pan drippings, until the meat is meltingly tender and beginning to pull away from the bone, another 2½ to 3 hours. The meat should easily pull away from the bone and an instant-read thermometer inserted into the meatiest part of the shoulder should register 170° to 180°.
6. Meanwhile, make the cumin salt: In a small, dry skillet set over medium-low heat, lightly toast the 2 tablespoons cumin until just fragrant but not darkened, about 1 minute. Let cool slightly and transfer to a mortar (or spice grinder) and crush to only lightly crack the seeds. Combine with the fleur de sel and set aside.
7. Place the lamb on a carving board, preferably one with a trough, to rest for 20 to 35 minutes. Pour off any excess fat from the roasting pan. Place the pan over two burners, add about ¼ cup of water and scrape and stir with a wooden spoon to dissolve any cooked-on bits. Using a meat fork and tongs, pull the lamb apart into serving pieces. Drizzle the pan drippings over the meat and serve with either individual bowls of cumin salt for dipping or one larger one to pass at the table.