01 - Combine salt, pepper, smoked paprika, garlic powder, onion powder, thyme, and brown sugar in a small bowl. Rub the spice mixture evenly over the entire surface of the trimmed beef brisket.
02 - Whisk together beef broth, Worcestershire sauce, apple cider vinegar, and Dijon mustard in a separate bowl. Pour the mixture into the slow cooker.
03 - Place the seasoned brisket in the slow cooker. Cover and cook on low heat for 8 hours until the meat is fork-tender and easily shreds.
04 - Transfer the cooked brisket to a cutting board. Use two forks to shred the meat, removing any excess fat. Skim the fat from the cooking juices, then return the shredded meat to the liquid to keep it moist.
05 - Whisk together mayonnaise, apple cider vinegar, Dijon mustard, sugar, salt, and pepper in a small bowl until smooth and well combined.
06 - Place green cabbage, red cabbage, grated carrot, and parsley in a large bowl. Pour the dressing over the vegetables and toss thoroughly to coat. Refrigerate until ready to serve.
07 - Brush the cut sides of the slider buns with melted butter. Toast in a skillet over medium heat or in a warm oven until lightly golden and crisp.
08 - Pile shredded brisket onto the bottom half of each bun. Top generously with coleslaw and add pickle slices if desired. Cover with the top bun and serve immediately.