Place the pork shoulder in your slow cooker (trim off any excess fat if you'd like, but it's not necessary).
Add chopped onion around the pork.
In a bowl, mix together BBQ sauce, apple cider vinegar, and brown sugar, then pour over the pork.
Cover and cook on low for 8-10 hours or on high for 5-6 hours, until pork is super tender and falls apart easily.
Remove pork from slow cooker and shred with two forks. Return shredded meat to the slow cooker and stir into the sauce. Serve on hamburger buns with coleslaw, or however your family loves it!