Place the lamb in a baking dish. Using the tip of a paring knife, make a series of 1/2 inch deep holes space 1-1/2 inches apart all over the lamb.
In a dry skillet over medium heat, toast the coriander, peppercorns, cumin, cloves, cardamom, and cinnamon for about 3 minutes, shaking the pan. Let cool. Grind to a fine powder in a spice mill. Crumble the saffron threads between hour thumb and forefinger and place in a small bowl with 1 tablespoon of hot water: infuse for 10 minutes. Place the spices in a blender jar. Add the onion, garlic, ginger, jalapeños, lemon juice, oil, and salt and blend to a smooth paste. Transfer to a bowl and stir in the yogurt. Pour over the lamb; cover and refrigerate for 8 hours.
Set up your Kalamazoo grill for spit-roasting and heat to medium-high. Thread the lamb onto a rotisserie spit, tighten the forks, and attach the spit to the motor. Spit-roast the lab until sizzling, browned, and cooked to your taste, about 1 hour for medium-well (170 degrees).
Transfer the lamb to a cutting board and let it rest for 15 minutes before carving and serving.
