In a small bowl, stir together the Dijon mustard, brown sugar, garlic, paprika, salt, pepper, and cinnamon.

Pat the pork butt dry with paper towels and rub the mustard mixture on all sides of the pork.

In a 6-quart or larger slow cooker, pour the apple cider vinegar. Add the sliced onions in a layer. Place the seasoned pork butt over top of the onions with the fat cap facing up.

Place the lid on the slow cooker and cook on LOW for 8 to 9 hours.

When the cooking time is up, use a slotted spoon to remove the pork and onions to a rimmed baking sheet or large shallow bowl. Using two forks, shred the pork and onions, adding some of the cooking liquid from the slow cooker as desired.

While the pork is cooking, place the 8 cups of water in a large pot over medium-high heat; bring to a boil.

Add the diced sweet potatoes and reduce the heat to medium to hold a simmer. Simmer, uncovered, until the potatoes are fork tender, about 18 to 20 minutes.

Remove the pot from the heat. Set aside 1/2 cup of the cooking liquid, then strain the potatoes in a colander. Return the tender sweet potatoes to the pot.

Add the butter and mash the potatoes, adding some of the reserved liquid back to achieve the desired consistency. Season with salt to taste.

While the pork is cooking, prepare the apple slaw. To a large mixing bowl, add the apple cider vinegar, oil, maple syrup, Dijon, minced garlic, salt, and pepper; whisk well to combine.

Add the coleslaw mix, sliced apple, and green onions, and toss to coat with the dressing. Refrigerate until ready to serve.

To serve, divide the mashed sweet potatoes among serving bowls. Top each with a serving of the pulled pork and apple slaw.

If desired, garnish with chopped parsley.