Toast the peppercorns, mustard, cumin, coriander, and fennel seeds on a dry skillet at medium heat for a few minutes until aromatic. Remove from heat and allow them to cool.
Add to a spice grinder and grind to a medium coarseness. Combine with the rest of the spices to make the seasoning blend.
Preheat the smoker to 265°F. Season the pork shoulder on all sides generously with the rub. Place it in the smoker and let it cook for 3 hours undisturbed.
After 3 hours have passed, spritz with apple cider vinegar every 45 minutes for the next 5 hours.
When the fat cap has started to split and it’s been about 8 hours total, spritz one final time and wrap with heavy-duty aluminum foil. Increase the heat to 285°F in the smoker and cook for another 2 hours, or until it’s about 200°F internal temperature.
Remove the pork shoulder and keep it in the foil while it rests for an hour. Shred when ready to serve with the sauce and slaw on sandwiches.