Make Puree - In a food processor, add coarsely chopped onion half, garlic, chipotle pepper, avocado leaves, and beans. Process to a smooth paste and stir in ½ tsp salt. If using a blender, add a small amount of the bean liquid to facilitate blending.
Make Sauce - Heat the lard or oil in a saucepan over medium heat until rippling. Add the pureed bean mixture and cook, stirring constantly, for 2 minutes. Gradually stir in enough of the reserved bean-water liquid to thin the mixture to the consistency of heavy cream. Bring to a boil and then reduce heat to low to keep warm
Prepare Onion Relish - In a small bowl, combine the diced onion, ½ tsp salt, vinegar, and oregano. Set aside.
Assemble Enfrijoladas - In a small skillet just large enough to hold a tortillas, heat the vegetable oil over high heat. Fry tortillas one at a time for 30 seconds each. Transfer to paper towels to drain. Repeat. While they are still hot and pliable, fold each into quarter. Dip each into hot bean puree and place on a platter or 2 per plate. Sprinkle tops with onion relish and crumbled cheese. Serve immediately.