OPTIONAL: Soak the corned beef in a pot of water in the refrigerator overnight, make sure the beef is submerged completely. This pulls some of the salt cure out of the brisket.
Set-up your Lang cooker to 250º degrees F.
Drain the corned beef and pat dry with paper towels, let come to room temperature.
Combine the rub and mustard slather ingredients and set aside.
Lay the corned beef fat side down and lightly paint the meat side and the sides with the slather using a pastry brush, sprinkle on the rub. Turn the meat over and repeat the process.
Place it on your Lang and cook until the internal temperature is 195º to 205º degrees F. Approximately 8-10 hours.
Remove the pastrami, tent with foil and let it rest for at least 30 minutes.
Recipe by Lang BBQ Smokers Recipe Blog at https://www.langbbqsmokers.com/blog/homemade-smoked-pastrami/