Trim any excess fat on the lamb, then score the back side fat to help cook faster. Season the outside with ample amounts of salt. Using metal wire and cutters, secure the butterflied lamb to the asado cross around the legs and along the back.
Having built a medium-high temperature fire, place lamb over fire with the flayed side towards the fire. Cook at a consistent medium-high for 3-4 hours. Move lamb away from fire if it begins to burn.
While lamb is cooking, add all marinade ingredients to a cast iron skillet and let simmer over fire for 10 minutes. Baste lamb all over every 30 minutes of the cook to maintain moisture.
After 3-4 hours, carefully flip lamb with back towards fire and cook for another 2-3 hours. Internal temperature is anything over 145F, but ideally would be into the 180’s to 190’s as the lamb will melt off the bone here. Continue to baste throughout.
Pull lamb off of asado cross, slice and serve!