Start a charcoal or gas grill. Gas should be set to high, charcoal is ready when the coals are red but entirely covered with gray ash and you can only hold your hand above them, about 5 or 6 inches away, for about 4 to 5 seconds before it becomes too hot.
Clean and season the grill by rubbing a quarter of a white onion over the grill, using a pair of tongs. Next, rub the beef fat, tallow or vegetable oil over the grill to further season it.
Working in batches if necessary, place the meat on the hot grill. Season the top of the meat generously with salt just before you put it on the grill, salted side up. Or salt it once it is on the grill.
Grill for 4 to 5 minutes, until the bottom has seared, deep grill marks have developed, and the meat juices have started rising and bubbling over the top. Flip over and grill for another 3 to 4 minutes for medium-rare. Remove the meat and place in a lidded dish or container. Cover while you grill the remaining meat. Allow the meat to rest for 5 minutes before cutting.
One by one, take out the pieces of meat and cut into 1/2-inch dice or slice thin. Place the cut up meat back into the dish and cover to keep warm as you work.
Reduce the heat of the grill to medium if using gas. If using charcoal, set the tortillas on a cooler part of the grill. Heat the flour tortillas for about 1 minute per side, until puffed, lightly browned and completely heated through. Keep warm in a clean kitchen towel or tortilla warmer and bring to the table.
To assemble the tacos:
Spread some refried beans on the middle of a warm flour tortilla, add a generous amount of meat, then top the meat with salsa and guacamole to taste. Your taco should be so full that it’s difficult to close it!