Heat a large frying pan to high heat. Add the canola oil and swirl it around in the pan to coat the bottom evenly. Sear the pork shoulder until you get a nice brown/golden color on all sides.
Once it’s golden, place the pork in a slow cooker with all of the other pulled pork ingredients.
Cook at high temperature in the slow cooker for approximately 4 hours, covered (until tender and falls apart).
For the special sauce, mix the mayonnaise with the apple cider vinegar, cumin powder, sugar, salt and pepper. Mix well and refrigerate until ready to use.
Remove the pork from the slow cooker to a cutting board. Shred the pork with two forks.
Place all of the cooking liquid and the vegetables in a blender and pulse until blended. (Make sure to open the vent hole in the lid and cover with a kitchen towel to release the steam without any splashing.)
Return the pork to the slow cooker. Pour the mixture over the shredded pork and combine well. Adjust salt and pepper as needed.
Toast the buns. Assemble the burgers with the watercress, tomato slices, pulled pork and special mayo. Serve!
Notes
Store leftovers covered in the fridge for up to 4 days.
To freeze it, let it cool (no longer than 2 hours) and place it in double-bagged freezer bags with the air squeezed out to avoid freezer burn or you can vacuum seal it if that option is available to you. Thaw in the fridge overnight.