1) In a 4 1/2 to 6-quart slow-cooker, stir onion, ketchup, vinegar, brown sugar, tomato paste, paprika, Worcestershire, mustard, salt, and pepper until combined. Add pork to sauce mixture and turn to coat well with sauce.
2) Cover slow-cooker with lid and cook pork mixture on low setting 8 to 10 hours or until pork is very tender.
3) With tongs, transfer pork to large bowl or platter. Turn slow-cooker to high; cover and heat sauce to boiling to thicken and reduce slightly.
While sauce boils, with 2 forks, pull pork into shreds. Return pork to slow-cooker and toss with sauce. Serve on bun.