Rinse the pork butt, pat dry and place in a pot with edges 1-2 inches taller than pork. In a blender, combine the onion, vinegar and salt. Blend for 30 seconds on high. Pour the vinegar mix over the pork butt. If the vinegar does not cover the meat, add water until liquid covers it. Marinate the pork for 8-12 hours. It need not be refrigerated. Remove excess liquid from pork and transfer it to a 9x12 glass baking dish. Rub the pork with the mustard and pepper.  Place in grill, uncovered, and cook via indirect heat for 2 hours at 350 degrees.* Take the dish out of the grill and the pork out of the dish and wrap the pork in aluminum foil, ensuring that the foil covers the pork entirely, with no holes for heat to escape.  Place the pork back in the baking dish and put the dish back into the grill.
Cook the pork via indirect heat for another 2 hours at 350 degrees. Note: The pork is ready when the bone can be taken out with minimal tug; the bone should  separate from the meat effortlessly. Shred the pork. Optional, for crispy carnitas: Fry the pork in medium heat in a saute pan until dark brown. Serve pork inside warm tortillas with chopped onion, cilantro and lime juice.

*Cooking time varies depending on size of pork butt or shoulder. Plan to cook the pork for 30 minutes per pound.

Serves 10-12.