Brown the pork loin roast in olive oil in a skillet.
Transfer the roast to a slow cooker, surrounding it with sliced onion.
Mix apple cider vinegar, soy sauce, honey, brown sugar, garlic, salt, and red pepper flakes (if used) to create a marinade.
Pour this marinade over the pork and onions in the slow cooker.
Add pineapple chunks around the roast.
Cook on low for 6-8 hours or high for 4-5 hours until pork is tender.
Optional: Thicken the sauce with a cornstarch slurry if desired.