This is my all-time favorite sandwich, and lucky for me, it's from XOCO. The sandwich features shredded braised short ribs piled high on an artisan baguette, topped with caramelized onions, our custom blended Jack cheese, homemade pickled jalapenos and our roasted green salsa on the side. I've managed to come very close to the original with this version using a slow cooker. So if you can't get to XOCO, why not give this version a try, and see if it doesn't become one of your favorite sandwiches too.
Trim off any excess fat from the tops of the short ribs and generously salt all sides. Heat the oil in a stovetop-rated slow cooker liner (see testing notes below) over medium-high heat. (If your slow cooker liner isn’t made from a material that can be used on a stovetop, use a very large (12-inch) non-stick skillet.) Saute the short ribs in batches until well browned on all sides, about 6 minutes per batch. You can hold the finished ribs on a rimmed baking sheet until all the batches are done. Whether you're using the slow cooker insert or a skillet, pour off all the fat from the bottom of the pan and reserve.
Place the browned short ribs into the bottom of the slow cooker. Roughly chopped 2 onions and add them to the pot along with the smashed garlic cloves, beef broth, Worcestershire and soy sauces. Place the lid on top and set the slow cooker on high and the timer for 6 hours.
Slice the remaining 3 onions into 1/4-inch slices. Using the reserved pan dripping from browning the short ribs, add 1 1/2 tablespoons to a very large (12-inch) skillet set over medium heat. Once hot, add the onions and stir to coat. Cover the pan and cook for 15 minutes until the onions have softened. Remove the cover and stir occasionally for about 30 more minutes until the onions have caramelized. Cool and refrigerate until you're ready to assemble the tortas.
Preheat the oven to 375 degrees.
Once the meat is done, remove it to a rimmed baking sheet. When it has cooled down enough to handle, discard the bones and shred the meat taking care to remove any excess fat. Strain the juices that are left in the bottom of the slow cooker and skim off any fat. Place the shredded meat and 3/4 cup of the strained juices in a 10-inch skillet. (If you're working ahead, you can wrap the meat and juices and refrigerate or freeze for later use.) Place the skillet over medium heat and stir until the meat is heated through, about 4 minutes. Taste and season with additional salt, usually about 3/4 teaspoon.
Slice the baguettes lengthwise and place on a rimmed baking sheet. Add a layer of meat to the bottom of each of the sandwiches, top with the caramelized onions and the shredded cheese. Replace the tops and place in the oven for 10 minutes or until the cheese has melted and the tortas are warmed through. Remove the tortas to a platter, lift the tops off and add the pickled jalapenos and a drizzle of the salsa. Replace the tops and serve immediately.
Testing Notes: I used the All-Clad Deluxe Slow Cooker with the non-stick cast aluminum insert which allowed me to brown the short ribs on the stove top before placing the insert into the slow cooker. The cast aluminum surface heated very evenly and it saved me the clean up of having to use another skillet for browning. It has the added benefit of a stainless steel body which makes it attractive enough to use as a serving piece. It's definitely an investment, however, if you do a lot of slow cooking, it's worth the price.