Pat the pork roast dry with paper towels. Season all sides generously with salt, black pepper, cinnamon, and dried thyme.
Heat the olive oil in a large skillet over medium-high heat. Add the pork and sear for 2–3 minutes per side, turning until nicely browned. Transfer the roast to the slow cooker.
Whisk together the apple cider, chicken broth, Dijon mustard, and brown sugar. Pour the mixture over the pork, then add the sliced onion, apples, and minced garlic around the roast.
Cover and cook on LOW for 6–7 hours, or until the pork is tender and reaches at least 145°F in the thickest part. Avoid lifting the lid frequently during cooking.
Carefully transfer the cooked pork to a cutting board and loosely cover it with foil. Let it rest for 10–15 minutes before slicing.
Strain or spoon the cooking liquid into a saucepan if desired. Whisk together the cornstarch and cold water, then stir the slurry into the warm cider liquid. Simmer for 2–3 minutes until slightly thickened.
Slice the rested pork and arrange it with the cooked apples and onions. Spoon the warm apple cider sauce over the top and serve immediately.
