COMBINE all of the spice rub ingredients in a small bowl. Sprinkle the rub evenly along the sides and top of the pork roast.
ADD the onion slices to the bottom of a six-quart slow cooker. Place the seasoned pork roast on top of the onions. Then, add the apple cider. (If the roast is too large, you may wish to cut it in half and place the halves side by side in the slow cooker.)
COVER and cook on high 7-8 hours, or on low for 8-10 hours, until the meat is tender enough to be easily shredded with a fork.
TURN OFF slow cooker once the roast is done. Carefully transfer the pork to a large platter and allow it to rest a few minutes.
POUR the cooking liquid through a fine-mesh strainer into a large bowl. Place one cup of the liquid back into the slow cooker; discard the remaining liquid and onion.
SHRED the pork using two forks. Then, return it to the slow cooker. Toss well with the reserved cooking liquid. Season to taste with additional sea salt and pepper, if desired.
SERVE the pork with your favorite Whole30-compatible coleslaw and a side of mashed sweet potatoes. (Plus, save the leftover pulled pork to make even more tasty meals, as noted below.) Enjoy!