Short ribs: cut short ribs between the bone until they are small enough size to fit into a baking dish.
Combine brown sugar, ketchup, apple cider vinegar, sriracha, bourbon, Dijon mustard, Worcestershire sauce, paprika, garlic, and onion powders in a mixing bowl and whisk to combine.
Pour your sauce into a baking dish and nestle your short ribs in the sauce, flipping them over a few times to ensure they are coated.
Preheat your oven to 300 degrees.
Cover your baking dish with aluminum foil and bake short ribs for 3 hours, removing the foil after 2.5 hours so the meat will brown. You can cook these for an additional 30 minutes if you want even more tender meat.
Let the meat cool for 10 minutes and then remove all of the bones from the meat. You can shred the meat up at this point or do it later when you are about to build your sandwich. After the bones are removed, you can build your sandwich immediately or if you want to eat it later you can package the short ribs and sauce in the fridge.
If you eat it later, you will probably want to skim off some of the fat left behind from the ribs in the sauce. Let the sauce sit for a few minutes and the fat will come to the top and you can strain it off with a spoon. If you let the sauce cool fully, the fat is much easier to see and skim and remove.
Pickled red onions: slice a large red onion through the stem/root. Remove peel on one half and thinly slice. Save the other half of the onion for another recipe.
Add your thin slices of onion to a bowl with the rice wine vinegar. Sprinkle with granulated sugar, a pinch of kosher salt, and a pinch of MSG if using. Stir everything together and leave on your counter for at least an hour, stirring occasionally.
At first, all the onions will not be inside the liquid, but as things soften and the onion releases more liquid, everything will eventually become submerged.
Add to a container with a lid and store in the refrigerator for a couple of weeks.
Sandwich assembly: place a medium pan over medium heat.
Add butter to one side of both slices of bread. Place each slice, butter side down into the pan.
Add shredded cheese on top of both slices of bread and cover the pan for 2 minutes to help the cheese start to melt.
Add a few spoonfuls of short ribs and pickled onions on top of the cheese on one slice of bread and then carefully flip the other slice on top to create the sandwich.
Grill the sandwich until both sides are golden brown. Start with 2 minutes per side and then flip. Keep the lid on the pan when you can so that the steam will help to melt everything. Once both sides of the sandwich are golden brown, remove it from the pan and allow the sandwich to rest on a rack for 1 to 2 minutes.
Serve and enjoy.