Put onion in a bowl and cover with cold water. Set aside.
Pour beans into a blender and add salt to taste, plus enough bean broth to cover by about 1 in. Blend until very smooth.
Preheat oven to 200° and set a serving platter inside to warm. Line a rimmed baking sheet with paper towels. Put lard in a medium (not nonstick) frying pan over medium-high heat.
When lard is melted and hot but not quite smoking, slip a tortilla into it and fry until slightly crisp around edges but still flexible. Turn over once (clasp between 2 "nested" spoons to avoid tearing), 30 seconds to 1 minute total. Transfer to paper towels to drain. Repeat with remaining tortillas, layering between paper towels as needed. Put filled pan of tortillas in oven to keep warm.
Pour out all but 1 tbsp. lard from tortilla-frying pan. Add chorizo and fry over medium-high heat, stirring occasionally, until crisp around the edges (if using Mexican chorizo, fry until broken up and well browned), 8 to 10 minutes. Transfer to a bowl.
Reduce heat to medium-low, pour bean sauce into pan, and heat, stirring, until steaming; then reduce heat to low. Sauce should be no thicker than heavy cream; add more hot bean broth or water if necessary to thin it.
Using spoons again to grip tortillas, dip one tortilla at a time into bean sauce to thoroughly coat both sides. Lay on warmed serving platter and cover half of one side with a few pieces or a spoonful of chorizo. Fold over. Repeat with remaining tortillas, bean sauce, and chorizo. Drizzle with a little more sauce, then top with onions, queso fresco, and crema. Serve with pickled chiles.
*Find bayo chocolate beans at ranchogordo.com. For the lard, use the soft, brown, fresh Mexican-style lard from the deli counter at Latino markets, or any other fresh (nonhydrogenated) lard; vegetable oil will work, but it won't taste the same. Find crema and queso fresco at Latino markets too. Queso fresco is tastier and crumblier if removed from its package, blotted dry, and allowed to sit, uncovered, 1 to 2 days in the refrigerator.