- Heat the pit. It's a good idea to build a fire in the pit the night before you're going make the barbacoa to preheat the bricks and earth that surrounds them. Build a rip-roaring fire - keep it going for about 3 hours - then let it burn out and cover the pit to trap residual heat.Early on the morning of your party, uncover the pit and build another huge fire in it. Keep adding wood so that the fire stays very hot (700 to 800 degrees) for at least 3 or 4 hours.
- Make the adobo. Roast 6 cloves of the garlic in a large dry skillet over medium heat, turning regularly, until soft and blotchy black in spots, about 15 minutes. Meanwhile, heat the oil in another large skillet over medium-high. A few at a time, oil-toast the chiles for a few seconds on each side, until noticeably darker and toasty smelling - certainly not smoking and blackened. Remove the chiles to a large bowl, pour on 3 cups of hot tap water, weight with a plate to keep them submerged. Soak 20 minutes.Peel the roasted garlic and place in a blender along with the chiles (and their soaking liquid), vinegar, spices, salt and sugar. Blend to a smooth puree. Press through a medium-mesh strainer (to catch strain seeds and unblended skin) set over a large bowl
- Marinate the lamb and set up the roasting pan. Smear the red chile marinade over the lamb. Set 4 3- to 5-inch heat-proof supports (empty tin cans, custard cups or ramekins) near the 4 corners of your huge braising pan. Peel the remaining garlic, roughly chop it and scatter it over the bottom of pan, along with the onion, carrot and herbs. If you have the innards (heart, kidney and liver) add them to the pan as well. Set a rack on the supports (an oven rack, grill grate or large cooling rack are good choices). Cover with 2 of the banana leaves and half of the avocado leaves. Lay in the pieces of marinated lamb keeping them in as close to a single layer as possible. Cover with the remaining avocado leaves and 2 pieces of banana leaves. Carry to the pit.
- Cook the lamb. The fire should be burning very, very hot just before you lower in the lamb. Use a shovel or fireplace tongs to remove all the burning logs (but not the coals) from the pit. Pour 1/2 gallon water around the edges of the lamb. With the help of another (strong) person wearing oven mits, lower the braising pan of meat onto the coals in the very hot pit. Immediately cover the pit. Transfer the smoldering logs to the middle of the cover. Shovel enough dirt around the edges of the cover to completely seal the pit (when complete, you should see no smoke or steam escaping). Shovel more dirt (around the smoldering logs) over the cover to a depth of about 2 inches, to trap the heat.Let the lamb cook for about 6 hours. If everything gone right, after 6 hours, the temperature of the cover, once you've swept the dirt back with your hand, will be about 175 degrees - so hot that you can only touch it for a few seconds.
- Unearth the lamb and serve. Shovel and sweep all the dirt off the cover (I shovel it into a garbage can that I cover and store until the next time I use the pit). Uncover and lift out the braising pan. Remove and discard the top set of leaves. Transfer the meat to large serving platters or roasting pans, pulling off and discarding the bones (and the fat and skin, if you're so inclined) as you go. Keep warm in a low oven.Discard the leaves that were below the meat. Remove the rack and supports. Ladle the soup into a large soup pot. Fish out the garlic and herbs and discard. (Fish out and cut up the pieces of innards if you used them, then return them to the soup.) Let stand a few minutes for the fat to rise to the top, then skim it off. Taste and season the soup with salt, usually about 1 1/2 tablespoons. Keep warm over low heat.
When you're ready to serve, ladle the soup into small bowls for your guests to enjoy as a first course. Then, sprinkle the meat with coarse salt and set it for everyone to enjoy with warm corn tortillas, salsa, guacamole and black beans.