Spread onion on bottom of crockpot. Layer pork slices on top of onion. Pour sauce over meat. Cover and cook on high heat about 4 hours, until meat shreds easily.

Remove meat from crockpot and place on cutting board. Strain cooking liquid through fine-mesh stainer and discard solids. Shred meat and return to crockpot.

Add enough BBQ sauce to crockpot to moisten pork. Mix to coat. Serve with sauce on the side.