Prep: Prepare caramelized onions beforehand by sautéing 2-3 sliced white onions in a large saute pan in olive oil or butter over medium or medium-low heat for approximately 30 minutes or until soft and fully browned. Add a pinch of salt and sugar about halfway through cooking. Stir occasionally to prevent burning. Add water or more oil as necessary to prevent onions from drying out. Store in fridge for up to 3 days in a sealed container.
In a slow cooker, add beef, butter, hot sauce, beef broth, soy sauce, chipotles, garlic, cumin, and oregano. Cook on low for 6-8 hours or until the beef is tender and falls apart easily when shredded with a fork.
Using two forks, shred the beef into bite-sized pieces inside of the slow cooker. Toss the beef with the juices; cover and let the barbacoa soak up the juices for an extra 10 minutes or so.
Slice dinner rolls in half. Assemble sliders by spreading a thin layer of mayo-ketchup onto the bottom half of each roll. Top with cole slaw (optional), followed by a generous spoonful of barbacoa, caramelized onions, and pickles, then close the slider.