This is kind of a mashup of a few different recipes, mostly inspired by Mission Chinese and Serious Eats.
This requires a smoking gun if you plan to cook this in a house or apartment without access to a smoker. I prefer to use red oak as my wood chips of choice as an homage to my hometown of Santa Maria, California.
If need be, cut your brisket into pieces of the appropriate size to fill the bags for your vacuum sealer
Make the rub by mixing all of the garlic, cayenne, pepper, cumin, salt, sugar, and celery seeds in a bowl
Slather the mustard all over the brisket to fully coat it, then add the rub, ensuring you have a perfectly uniform coating of rub all around the meat
Add the brisket to a bowl and cover with plastic wrap
Grab your smoking gun and favorite wood chips, and smoke the brisket for 1-2 minutes, keeping the bowl covered with plastic wrap to seal in the smoke while it is smoking
Add your brisket (or briskets if you cut it up) to bags, and seal using your vacuum sealer
Heat a water bath to 155 F (68 C) and add the brisket. Leave in the water bath for 36 hours (1.5 days)
Remove the brisket and allow it to cool, then open the bag and reserve the liquid to use in making your BBQ sauce
Set your oven to 275 F (135 C) and turn on convection if you have it. If you don't have a convection setting, use 300 F (150 C)
Place the brisket on a wire cooling rack on top of a half sheet pan, and roast in the oven for at least 2 hours to form a dark bark around the exterior
Rest for a bit to allow it to cool, then slice and serve with pickles, white bread, and BBQ sauce