01 - Mix 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 - Combine 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 heat for 8 hours until the brisket becomes very tender and easily shreds with a fork.
04 - Transfer the cooked brisket to a cutting board. Use two forks to shred the meat, discarding any excess fat. Skim the fat from the cooking juices and return the shredded brisket to the juices to keep it moist.
05 - Toss green cabbage, red cabbage, carrot, and parsley together in a large bowl. Whisk mayonnaise, apple cider vinegar, Dijon mustard, sugar, salt, and pepper in a separate bowl. Pour the dressing over the cabbage mixture and toss until evenly coated. Refrigerate until ready to serve.
06 - Brush the cut sides of the slider buns with melted butter if desired. Lightly toast them in a skillet or oven until golden.
07 - Pile the shredded brisket onto the bottom half of each bun. Top generously with coleslaw and add pickle slices if using. Cover with the top bun and serve immediately.