Trim the fat cap of the brisket to about 1/2" thick and remove any silverskin.
Brush the brisket on all sides with mustard. Sprinkle with Cowboy Rub and rub into brisket ensuring full coverage.
Place brisket on a wire rack over a sheet tray. Cover with aluminum foil. Refrigerate overnight, 10 to 12 hours.
Set oven to 275°F. Cook brisket until it reaches 180°F, about 6 hours. Uncover and until exterior is crisp and the internal temperature stays at 200°F for a minimum of 1 hour.
Transfer the brisket to a clean cutting board. Cover loosely with foil and let rest for 30 minutes before slicing across the grain and serving.
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