Place the sliced onion in the bottom of the slow cooker. Add the pork shoulder on top.
Pour the apple cider over the pork. Sprinkle the minced garlic, salt, pepper, and cinnamon across the meat.
Cover and cook on low for 8 hours or until the pork shreds easily with a fork.
While the pork cooks, preheat the oven to 400 degrees. Toss the carrots, parsnips, and sweet potatoes with the olive oil, thyme, and a pinch of salt.
Spread the vegetables in a single layer on a baking sheet. Roast for 30 to 35 minutes, stirring once, until the edges are golden and tender.
Shred the pork directly in the slow cooker and mix it with the cooking juices. Serve the pulled pork alongside the roasted vegetables.