Rub the chuck roast heavily with the Cajun seasoning, black pepper, and garlic powder. Sear it in a high-heat pot with 2 tablespoons of oil for 4 minutes per side until heavily browned.
Transfer the meat to a slow cooker or a heavy Dutch oven.
In a bowl, whisk together the beef broth, the dry onion soup mix packet, the dry brown gravy mix packet, the Worcestershire sauce, and the hot sauce. Pour this entire mixture completely over the roast.
Cook on Low for 8 to 10 hours. The secret to the NOLA Poboys texture is time; the long cook completely breaks down the chuck roast fat into the gravy.
Remove the meat, thinly slice it, and ladle the rich onion-brown gravy heavily over the top.
Shred the meat directly into the pot using two forks so it floats inside the heavy, dark au jus drippings.
Slather your bread with mayo, pile the hot beef on, and top it with plenty of lettuce, tomato, and pickles.