Roughly chop the onions and place them in the bottom of a slow cooker or Instant Pot.
Put the pork shoulder on top of the onions.
Pour the water and fresh pineapple juice over the pork.
In a small bowl, whisk together the minced garlic, mustard, ketchup, cumin, paprika, salt, and apple cider vinegar.
Pour the spice mixture over the pork.
Cook in a slow cooker on low for 7 hours or on high for 5 hours. Cook in an Instant Pot on high pressure for 45 minutes and allow pressure to release naturally.
With one hour of slow cooking time left or after pressure releases in Instant Pot, remove pork to a cutting board and use two forks to shred.
Return the pork and add the diced pineapple to the slow cooker to finish cooking or to the Instant Pot to heat through.