Place sliced onions on bottom of slow cooker.
Set pork shoulder on top of onions.
In small bowl, combine cider, cinnamon, garlic powder, and salt.
Pour over pork.
Cook on low for 8-10 hours, until pork easily shreds.
To make BBQ Sauce:
Combine cider, cinnamon, and barbecue sauce in small saucepan over medium heat.
Bring to a simmer and let cook 10-15 minutes, until mixture has reduced to the consistency of barbecue sauce.
To make coleslaw:
Combine all coleslaw ingredients and mix well.
Refrigerate until ready to use.
Can be made up to one day in advance.
Serve pork on buns (I used pretzel buns!) topped with coleslaw and barbecue sauce.