For serving: brioche or hamburger buns, coleslaw, sliced dill pickles, potato chips
Place brisket in a 4 to 6 quart slow cooker, fat side up. Add the onion on top. Whisk together the apple cider vinegar and BBQ sauce and pour over brisket. Cover and cook on high until brisket is tender, about 5 hours.
Let meat rest about 20 min before slicing and reserve the sauce in the slow cooker. Toast the rolls and add sliced brisket to the bottom of each roll. Add a spoonful of coleslaw and dill pickles if desired.
Serve sandwiches drizzled with reserved sauce and potato chips