Season lamb very generously with Sel Magique; place on a wire rack set inside a rimmed baking sheet.
Toast peppercorns and coriander seeds in a dry small skillet over medium heat, tossing often, until fragrant, about 3 minutes; let cool. Finely grind in spice mill or with mortar and pestle; transfer to a small bowl. Mix in paprika, cinnamon, cardamom, cloves, and nutmeg, then rub all over lamb. Chill uncovered at least 4 hours and up to 1 day.
Let lamb sit out on counter at room temperature 1 hour before roasting.
Place a rack in middle of oven; preheat to 275°. Transfer lamb to a roasting pan and pour in 3 cups water. Cover tightly with foil and cook, turning lamb over and rotating pan halfway through, until meat is almost falling off the bone, 3–3½ hours. Remove lamb from oven and turn up heat to broil (or to highest heat if you have a drawer-style broiler). Discard foil, transfer lamb to a cutting board, and pour off liquid in pan. Return lamb to pan; roast, uncovered, until nicely browned, about 5 minutes.
Meanwhile, toss onion, lemon juice, and sumac in a small bowl with your hands, squeezing onion a bit to soften.
Just before serving, toss mint, parsley, and a pinch of Sel Magique into onion mixture. Serve lamb topped with onion salad. Use your favorite Sel Magique salt blend as needed.