The key to success in this recipe is time. Don’t try and rush the brisket by turning up the heat—that will only dry it out and make it tougher. Let it go low and slow until the meat is shreddable, and if you’re not sure if it’s ready, let it go a bit longer.
Remove brisket from its packaging and pat dry with paper towels. Lay brisket flat on your cutting board so that one of the longer sides is closest to you and layer of white fat is facing down. Take a close look at the meat itself—you’ll notice that there are long “grains” (thread-like muscle fibers) running left to right across length of brisket. You are going to “slice against the grain,” which means positioning your knife blade perpendicular to the direction of the fibers, into 2"-wide pieces. You do this so that when you go to shred the meat at the end, the fibers don’t stretch a mile long and they’re a manageable length for sandwiches.
Place brisket halves in a large bowl and sprinkle with salt, brown sugar, and cumin. Use your hands to really massage the meat and work in all the salt and spices so every little spot is coated. Get in there! Set brisket aside for a moment.
Mix onion, garlic, vinegar, soy sauce, Worcestershire, ⅔ cup Sriracha, and 1 cup water in a large Dutch oven or stockpot.
Using tongs, nestle brisket into pot so fattier sides are facing up and pieces fit together in a single layer. The meat should be barely submerged in liquid, so add a little more water if that’s not the case (depending on size of your pot).
Bring to a simmer over medium heat, then reduce heat to achieve a very gentle simmer.
Cover pot and cook brisket, peeking inside occasionally to make sure heat isn’t too high or low—you want a couple of areas in the pot to be gently bubbling. If bubbles are roiling all over the place, reduce heat. If you see no bubbles at all, increase slightly. If liquid drops below top of meat, add a little more water to keep barely submerged.
After 2 hours, check meat for doneness. Poke a fork into a few pieces and twist the tines to see if the meat will easily separate into shreds. If not, that means it’s still tough. Keep cooking, checking again every 15 minutes or so. This could take up to an hour more.
Turn off heat and position lid so that it’s cracked open a bit. Let brisket cool until no longer steaming hot, about 30–40 minutes. Cover and chill overnight.
The next day, remove pot from refrigerator and uncover. There will be an layer of solidified orange fat on the surface (the color is from the Sriracha!). Using a fork, lift off fat in large pieces and discard.
Reheat brisket in pot over low until meat is warmed all the way through, 30–40 minutes. Transfer meat to a rimmed baking sheet. If meat is too hot to touch, let cool for a few minutes until you can handle it.
Using your fingers or 2 forks, separate muscle fibers into shreds and return to pot. Remove any pieces of fat and discard.
Return pot to low heat and bring liquid to a gentle simmer, tossing meat with tongs to encourage it to soak up juices. Add more Sriracha to taste.
Serve over toasted buns with even more Sriracha, if desired.