Trim any excess fat on lamb; score back side fat to help cook faster. Season outside with ample amounts of salt. Using metal wire and cutters, secure butterflied lamb to the asado cross around the legs and along the back.
Over a medium-high temperature fire, place lamb over fire with flayed side towards the fire. Cook at a consistent medium-high for 3 to 4 hours. Move lamb away from fire if it begins to burn.
While lamb is cooking, in a cast iron skillet add garlic, rosemary, thyme, lemons, beer, onion, mint, vinegar, oil and red pepper flakes; let simmer over fire for 10 minutes. Baste lamb all over every 30 minutes of the cook to maintain moisture.
After 3 to 4 hours, carefully flip lamb with back towards fire; cook for another 2 to 3 hours. Internal temperature is anything over 145°F, but ideally would be into the 180°F’s to 190°F’s as the lamb will melt off the bone here. Continue to baste throughout.
Pull lamb off of asado cross, slice and serve.
USDA recommends the following time and temperature parameters
Internal temperature of 160°F
All Other Cuts
Internal temperature of 145°F, with a 3 minute rest