2 tbs. kosher salt
2 tbs. dark brown sugar
1 tbs. black pepper
1 tbs. chili powder
3 tbs. smoked paprika
2 tsp. dry mustard
2 tsp. garlic powder
2 tsp. onion powder
1 tsp. cornstarch
1/2 tsp cayenne pepper
1/2 tsp. celery seeds
1 (7-8 lbs with bone) pork shoulder or pork butt
2 mediums onions, quartered
1 1/2 cups water
2 tbs. Worcestershire sauce
Preheat the oven to 500 degrees F. Place a wire rack inside of a baking sheet and set aside.
In a small bowl combine salt, brown sugar, black pepper, chili powder, smoked paprika, dry mustard, garlic powder, onion powder, cornstarch, cayenne pepper and celery seeds. Pat the pork dry and evenly coat with half of the spice rub mixture. Place the pork, fat-side up, on the wire rack and roast until it is sizzling. About 10 minutes.
While the pork is cooking, place the onions in a large slow cooker with the remaining rub, water, vinegar and Worcestershire sauce.
Transfer the pork to the slow cooker. Cover and cook on low until the pork is soft enough to pull apart with a spoon, about 14-16 hours.
Transfer the pork to a large bowl and let it cool enough to handle. Pull the meat apart into large chunks or shred with a fork. Discard any bone or fat.
Using a fat separator and a strainer, pour the juice from the slow cooker over the strainer into the fat separator to retain the liquid. Pour the defatted liquid over the meat and serve or cover for later.