01 - Combine salt, pepper, 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 brisket.
02 - Whisk together beef broth, Worcestershire sauce, apple cider vinegar, and Dijon mustard in a separate bowl. Pour the mixture over the seasoned brisket in the slow cooker.
03 - Cover the slow cooker and cook on low setting for 8 hours until the meat is fork-tender and easily shreds apart.
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 juices to maintain moisture.
05 - Toss green cabbage, red cabbage, carrot, and parsley in a large bowl. Whisk mayonnaise, apple cider vinegar, Dijon mustard, sugar, salt, and pepper in a separate bowl. Pour the dressing over the vegetables and mix thoroughly. Refrigerate until serving.
06 - Brush the cut sides of slider buns with melted butter and toast lightly in a skillet or oven until golden brown, if desired.
07 - Mound shredded brisket onto the bottom halves of the buns. Top generously with coleslaw and add pickle slices if using. Place the top buns and serve immediately.