Clean lamb and pat dry. Trim all excess fat and discard.

Prepare the Chipotle Marinade: In a blender, combine all ingredients and puree to a smooth, thick consistency. Set aside 1/2 cup and rub remaining marinade over lamb. Place lamb in refrigerator to marinate overnight.

Preheat oven to 375°F.

Place a wire rack on top of a flat sheet pan and place lamb atop. Roast, uncovered, for 3 to 3.5 hours. Remove from oven and let rest 15 to 30 minutes before slicing. Reserve lamb drippings for Chipotle Lamb Sauce.

Prepare the Chipotle Lamb Sauce: In a saucepan, heat drippings over medium-high heat. Once hot, add reserved marinade and heat 2 minutes. Stir in stock and reduce for 3 to 5 minutes to a gravy-like consistency. Season with salt.

To serve, slice lamb and serve with sauce drizzled over the top.