Pat the pork tenderloin dry and season it all over with the salt, black pepper, dried thyme, and cinnamon.
Heat the olive oil in a skillet over medium heat. Sear the pork on all sides until lightly browned, about 2 minutes per side.
Place the sweet potato chunks and sliced onion in the bottom of the slow cooker. Add the minced garlic on top.
Set the seared pork tenderloin on the vegetables. Pour the apple cider over everything.
Cover and cook on low for 6 to 7 hours, until the pork reaches an internal temperature of 145 degrees and the sweet potatoes are fork tender.
Remove the pork and let it rest for 5 minutes before slicing. Spoon the sweet potatoes and onions onto plates, add the sliced pork, and drizzle with some of the cooking liquid. Garnish with fresh thyme sprigs.