'Discovered' this by accident when starting the pork butt about 10 p.m. one night (the butt was still partially frozen). Started with the basics, but decided to add the stone ground mustard and the orange juice on a whim. Husband loves it, so I must have been inspired.
Place pork butt in a slow cooker with the fattier side facing upwards. Pour water and orange juice into the slow cooker. Season pork and liquid with salt and pepper.
Mix garlic and mustard together in a small bowl; spread over the top of the pork butt. Sprinkle onion over the pork.
Cook on Low, turning the butt a few times, until the meat falls apart easily when pulled with a fork, 6 to 8 hours.
Remove pork from slow cooker; allow to sit until cool enough to handle. Trim away and discard any visible chunks of fat. Shred remaining pork into strands. Return shredded pork to the slow cooker; stir. Remove portions of pork from slow cooker with a slotted spoon to serve.