Place the black beans and the onion in a large soup pot or casserole and add enough water to cover by at least 2- to 3-inches. Bring to a rolling boil over high heat and cover with a lid, leaving it slightly open. Reduce heat to medium-low and cook for an hour to an hour and 15 minutes – making sure there is always sufficient water (if you need to add a cup, make sure it is boiling hot).
Once the beans are cooked and tender, add 1 tablespoon salt and a couple sprigs of fresh epazote or cilantro. Cook for another 15 minutes. Turn off heat.
In a hot comal or skillet set over medium heat, toast the avocado leaves and chiles de arbol for a couple of minutes until fragrant and browned, flipping as they toast. Remove from the heat. Break the leaves into pieces. Remove the stem from the chiles and break into pieces without discarding the seeds.
Working in batches if necessary, add the cooked beans and at least 1 1/2 cups of their cooking liquid (or add water if need be) to the jar of a blender, as well as the avocado leaves and chiles. Puree until a little chunky.
In a large skillet or casserole set over medium-high heat, heat the lard or vegetable oil. Once hot, but not smoking, add the chopped onion. Cook until translucent and edges are beginning to brown, about 6 to 7 minutes. Incorporate pureed beans and reduce heat to medium. Cook, stirring occasionally, until they thicken to your liking. I cook them for about 10 to 12 minutes.