Pat the pork roast dry and season generously with salt and pepper.
Heat olive oil in a skillet over medium-high heat.
Sear the pork roast on all sides until golden brown.
Transfer sliced onions and garlic into the slow cooker or pot.
Place the seared pork roast on top of the onion mixture.
Pour apple cider evenly over the roast.
Sprinkle ground cinnamon if using.
Cover and cook on low for 6–8 hours without frequently opening the lid, until the pork is tender.
Occasionally baste the pork with cooking liquid if accessible.
Once cooked, remove pork and let it rest for 10 minutes.
Simmer the remaining liquid to slightly thicken if desired.
Slice or shred pork and serve with the cider glaze.