Stir together the maple syrup, Dijon mustard, minced garlic, salt, and pepper in a small bowl.
Place the sliced onion in the bottom of the slow cooker. Set the pork roast on top and pour the maple-Dijon mixture over the meat, turning it once to coat evenly.
Pour the apple cider around the pork. Add the thyme sprigs on top.
Cover and cook on low for 7 to 8 hours until the pork reaches an internal temperature of 195°F and pulls apart easily with a fork.
About 30 minutes before the pork finishes, preheat the oven to 400°F. Toss the apple wedges with olive oil and a pinch of salt on a baking sheet.
Roast the apples for 20 to 25 minutes until they turn golden and soften slightly at the edges.
Remove the pork from the slow cooker and let it rest for 5 minutes. Slice or shred it, then serve with the roasted apples and a spoonful of the cooking juices.