Preheat your Z Grills pellet smoker to 225°F.
Mix the pepper, kosher salt, brown sugar, onion powder, garlic powder, paprika, mustard powder, and cayenne pepper.
Remove the membrane off the bone side of the short ribs is optional.
Rub the short ribs on all sides with mustard.
Liberally apply the seasoning mixture on all sides.
Place the short ribs in the smoker and smoke for 4-5 hours, or until the internal temperature reaches 203°F. Measure at a middle point of the beef that is away from the bone.
After the ribs reach the desired internal temperature, remove them from the smoker. Let them rest for 10 minutes, then serve.