Add the apple cider vinegar, sugar, salt, and pickling spice to a pot and heat. Whisk until the salt and sugar are dissolved. Bring to a boil. Place the sliced red onion in a non-reactive container. Pour the hot pickling brine over the onion. Chill and store for up to a month.
Using a heated griddle, pan, or oven, toast the pretzel buns and reheat the Thick Sliced Smokehouse Brisket.
Spread BBQ sauce on both sides of the pretzel buns.
To assemble, add pickle slices to the bottom of the bun, stack 3 or 4 slices of brisket, then top with pickled onion, and place the top of the bun.