In a small bowl, mix the brown sugar, black pepper, paprika, garlic powder, onion powder, coriander, mustard seeds, thyme, cumin, and red pepper flakes (if using). Rub the mixture all over the brisket, pressing it in so it adheres.
Place the brisket in the slow cooker. Pour the beef broth and apple cider vinegar around the brisket (not over it), and stir in the liquid smoke if using. Cover and cook on LOW for 8–10 hours, until the meat is fork-tender.
Remove the brisket and let it rest 10–15 minutes. For deli-thin slices, chill fully or overnight before slicing.
(Optional) For a classic crust, set the brisket on a rack and broil or bake at 400°F for 5–10 minutes until the spice crust sets.
Slice thinly against the grain. Serve on rye bread with Dijon mustard, or enjoy with your favorite sides.