Add the pork shoulder chunks to the slow cooker. Layer the sliced onion, sliced apples, and minced garlic on top.
Pour the apple cider over the meat and vegetables. Sprinkle the cinnamon, thyme, salt, and pepper evenly across the surface.
Cover and cook on low for 8 hours until the pork pulls apart easily with a fork.
About 30 minutes before the pork is done, place the sweet potato cubes in a large pot and cover with water. Bring to a boil and cook until the potatoes are soft, about 15 minutes.
Drain the sweet potatoes and return them to the pot. Add the butter and milk, then mash until smooth. Season with a pinch of salt if needed.
Shred the pork directly in the slow cooker and stir it into the cooking liquid. Serve the pork alongside the mashed sweet potatoes.