Cook's Note: Epazote is an herb that resembles tarragon. You can substitute with tarragon.
Soak the chickpeas in water overnight. Drain and set aside.
Preheat the oven to 325 degrees F (160 degrees C).
Score the skin of the lamb with a very sharp knife. Sprinkle with salt and pepper. In a large cast-iron casserole over high heat, sear the lamb on all sides in the olive oil. Transfer to a plate.
In the same casserole, add the onions and saute until translucent, about 5 minutes. Add the chickpeas, ancho chiles, garlic, epazote and water. Put the lamb back in the casserole. Cover with cactus leaves if using. Cover the pot. Cook in the oven until the meat is fall-off-the-bone tender, about 6 hours.
Just before bringing to the table, stir in chopped epazote. Serve with salsa, chopped onions, cilantro, lime wedges and tortillas.