Set a 10-inch skillet over medium heat. When hot, add the avocado leaves and turn every few seconds until they brown lightly and release their aroma, about 1 minute. Transfer to blender jar. Lay the chiles in the skillet and turn nearly constantly until they, too, are aromatic and lightly browned, about 1 minute. Transfer to the blender. Add the lard, drippings or oil to the skillet and, when hot, add the onion and whole garlic cloves. Stir regularly until richly brown, about 7 minutes. Scrape into the blender, leaving behind as much fat as possible. Add the beans with their juice and a little water if necessary to keep the mixture moving through the blades. Blend as smoothly as possible. Scrape into the skillet with the fat and cook over medium heat, stirring a few times, until the mixture has reduced to the consistency of soft mashed potatoes, 10 to 15 minutes. Taste and season with salt, usually about 2 teaspoons, depending on the saltiness of the beans.
These beans are wonderful (and common in Oaxaca) spread on crisp tostadas or Oaxacan tlayudas topped with fresh cheese (crumbled) or Oaxacan quesillo (pulled into strings) and roasted tomatillo salsa made with serrano chiles or the smoky chile pasilla oaxaqueño.