Make caramelized onions. Set a very large (12-inch) skillet over medium-high heat. When hot, add the bacon and cook, stirring almost constantly, until it has rendered its fat, 2 to 3 minutes. Add lard or oil if needed to coat the bottom of the skillet. Add the onion to the pan, sprinkle with the sugar and about ½ teaspoon salt, and stir to coat everything well. Cook, stirring regularly, until the onions begin to brown and soften, about 2 minutes. Reduce the heat to medium-low and cook, stirring from time to time, until the onions are a rich golden, about 5 more minutes. Use a slotted spoon to scoop the onions into a bowl, leaving behind as much fat as possible.
Sear the beef. Return the pan to medium-high heat. If there isn’t enough fat to cover the bottom of the pan, add lard or oil. Spread out the bistec (or its stand-in) on an easy-to-wash cutting board and sprinkle both sides generously with salt and the pepper. When the fat in the pan is really hot, 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. Chop into small pieces—nothing larger than about ¼ inch.
Finish the dish. Return the meat and onions to the pan and set over medium heat. Add the Worcestershire sauce (if you’re using it) and stir for a few seconds as it evaporates. Taste and season with additional salt if you think necessary. When the mixture is hot, start making tacos: on a hot tortilla scoop a portion of the bistec encebollado followed by a sprinkling of cheese and cilantro. Pass the salsa separately.