Rub brisket with barbecue seasoning and wrap completely in tin foil, with the fat side facing upwards.
Place wrapped brisket in shallow roasting pan and bake in a 325ºF for 3 to 4 hours, or until tender.
Once brisket is baking, combine the prepared mayonnaise, caramelized onions, black pepper and 1/2 cup of barbecue sauce.
Meanwhile, bring water, apple cider vinegar and 1/2 cup of barbecue sauce to a boil and pour over the sliced onions, arranged in a heat-stable container. Be sure to cover the onions completely.
Allow the pickled onions to sit out at room temperature for a minimum of 1 hour, as the hot liquid softens the slices of onions. Refrigerate afterwards.
Once the brisket meat can be pulled off easily, remove tin foil, increase oven temperature to 375ºF, and baste the brisket on either side with the remaining barbecue sauce every 10 minutes.
Remove brisket from oven and allow to cool 10 to 15 minutes before serving. Serve slices of brisket with barbecue mayonnaise and pickled onions as is, or construct brisket sliders with miniature burger buns.
Want more heat?! For those unafraid of the Fresno chili pepper, add in slices of a Fresno chili pepper to the sliced onions and pickle the entire mixture. Bring on the heat and full flavor!
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