For the lamb: Use paper towels to pat dry the leg of lamb. In a small bowl, combine the salt, cumin, cinnamon, allspice, ginger, black pepper and cayenne. Season all sides of the lamb with the spice mixture and place on a roasting rack in a roasting pan. Let sit at room temperature for 30 to 45 minutes or place in the refrigerator overnight and allow to sit at room temperature for 45 minutes before cooking.
Preheat the oven to 450 degrees F.
Drizzle the lamb with the olive oil, then roast for 15 minutes. Reduce the oven temperature to 350 degrees F and bake until the internal temperature is 140 degrees F for medium, about 1 hour and 15 minutes more. Allow to rest before serving, 15 to 20 minutes, then slice.
For the sauce: Meanwhile, in a small bowl, mix the harissa, yogurt and honey. Season with salt. Refrigerate until serving.
For the optional garnish: Garnish with the pomegranate seeds, mint, scallions, good olive oil and sea salt if desired. Serve with the harissa yogurt sauce.
If you prefer a less spicy sauce, decrease the harissa by 1 tablespoon and increase the yogurt by 1 tablespoon.