Sear the beef. Set a very large (12-inch) skillet or griddle over medium-high heat. Season the meat generously with salt on both sides. When the pan is hot, add 2 tablespoons lard or oil, then lay in as much beef as fits in a single layer. Let sear undisturbed on one side (it’ll take 1 ½ to 2 minutes to get deep, rich color), then flip and brown the other side. Repeat with the remaining meat (adding more lard or oil if necessary to coat the pan). As the meat is done, transfer it to a cutting board. Reduce the heat to medium and. If necessary, add more lard or oil.
Finish the mixture. Add the chorizo or longaniza and the onion to the pan. Cook, breaking the sausage into small pieces with a spoon or spatula, until the sausage is cooked through and beginning to brown, 4 or 5 minutes. While the sausage is cooking, chop the seared beef into small pieces—nothing larger than about ¼ inch. Scoop it into the cooked sausage mixture, along with the potato and spices. Cook, stirring and scraping the bottom of the skillet frequently, until the mixture comes together into a light hash consistency and is nicely browned, 4 or 5 minutes. Taste and season with salt, usually about ½ teaspoon (unless of you started with salty cecina) and scrape into a serving bowl.
Serve. Set out the meat mixture with warm tortillas, chicharrón if you are using it and the salsa for everyone to make their own tacos.