1. Preparing the grill and soup ingredients. About 30 minutes before cooking, prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and are medium-hot. Bank the coals to both sides of your grill to prepare for the indirect cooking. Position the cooking grate 8 inches above the coals and set an oven thermometer on the grate.
In a heavy roasting pan (one that holds a small turkey is the right size), combine the potatoes, carrots, onion, garlic, garbanzos and epazote. Position the pan in the center of the grill grate. Pour water into the pan to about 2 inches from the top (it’ll take about 6 to 8 cups). Nestle a V-shaped roasting rack into the watery vegetable mixture.
2. Cooking the meat. Generously smear Adobo Marinade over all sides of the lamb roast—you’ll need at least 1 cup. Lay avocado leaves on the roasting rack—at the bottom of the V, they’ll be skimming the surface of the water—to create a substantial bed for the roast. Lay the roast on top, then cover it with the remaining avocado leaves. Cover the pan tightly with aluminum foil. Cover the grill and cook, maintaining a moderately low temperature (between 250 and 300 degrees), checking every 20 minutes and adding charcoal as needed. Cook until the lamb is beautifully smoky-roasted and fall-apart tender (it’ll register about 170 degrees on a meat thermometer), about 2 1/2 hours. Be sure to periodically check the slow-simmering soup that’s capturing all those aromatic lamb juices to ensure the liquid level remains more or less the same; add more water if it’s needed.
3. Finishing the dish. Remove and discard the avocado leaves on top of the roast. With a couple of meat forks or spatulas, remove the roast to a banana leaf-lined, a heat-proof platter, breaking it into large chunks. Discard any avocado leaves that cling to the meat or remain on the roasting rack. Sprinkle with salt, cover loosely with foil and keep warm in a low oven.
Meanwhile, with the precision of a steady-handed circus performer, carefully remove the pan of soup from the bottom of the grill. Skim off the fat that is floating on the surface, then taste and season with salt, usually about 3/4 teaspoon. Stir in the cilantro and ladle into small, warm soup cups.
Serve each guest a cup of soup, and pass the meat, salsa and lots of warm tortillas for everyone to make delicious soft tacos.