Slow Cooker Pulled Pork

Ingredients
i medium onion, chopped
1/2 cup ketchup
1/3 cup cider vinegar
1/4 packed brown sugar
1/4 cup tomato paste
2 tbsp sweet paprika
2 tbsp worcestershire sauce
2 tbsp yellow mustard
1 1/2 tsp salt
1 1/4 tsp pepper
4 lbs boneless pork shoulder blade roast, cut into 4 pieces
12 sandwich buns

Mix together onion, ketchup, vinegar, brown sugar, tomato paste, paprika, worcestershire sauce, mustard, salt, and pepper.
Place pork pieces into slow cooker and pour mixture evenly over the top.
Cook on low heat for 8-10 hours.
Remove pork from slow cooker, leaving sauce.
Turn heat setting to high. Cook sauce alone to thicken slightly.
Shred pork and return to slow cooker once sauce is boiling.
Spoon mixture onto sandwich buns and serve.