Preheat the oven or smoker to 225 degrees F.
Trim the fat evenly across the top to 1/4-inch thick. Season the brisket liberally with the seasoning rub. Cook in the oven or smoker until the internal temperature on an instant- read meat thermometer reaches 175 degrees F, about 6 to 8 hours. Remove the meat from the oven or smoker and wrap in a double layer of aluminum foil to seal in the juices. Turn the foiled meat upside down and place it back in the oven or smoker for 1 more hour. Remove and let the meat rest for 20 minutes. Slice in 1/4-inch thick slices.
Serve with jalapeno peppers and/or onions on a bun or in a soft flour tortilla shell with fajita seasoning and sauce.