For tender and juicy pork, use of a pressure cooker is highly recommended. You can cook the pork anyway you prefer. There is a link in Recipe Notes for how I prepare pork shoulder/butt in a pressure cooker. Amount of pork is up to you, when I make this I use 1 1/2 kilos (3 lbs).
After the pork is cooked, place on a cutting board and use two forks to shred it, then place the pork in your slow cooker and set to Low setting.
Add BBQ sauce as needed to coat all the shredded pork. Leave the pork on Low for about 30 minutes to heat everything through.
When the pulled pork is good and hot, lay out a slice of rye bread, then add a slice of onion, as thick as you like it, add some pickle slices.
Scoop on some hot BBQ pulled pork.
Top that with sliced Provolone, as many slices as you like.
Top with another slice of rye and enjoy. (Great photo except for the fly on the sandwich, oh well, it happens.)