1. Heat the olive oil in a large skillet over medium heat. Add the pork loin roast and sear it on all sides until browned. This will help seal in the flavors and juices.
2. Transfer the seared pork loin roast to a slow cooker. Add the sliced onion around the roast. (If you want, you can also sear the onions before putting it in).
3. In a mixing bowl, combine the apple cider vinegar, soy sauce, honey, dark brown sugar, minced garlic cloves, salt, and crushed red pepper flakes (if using). Mix well to create a marinade.
4. Pour the marinade over the pork loin roast and onions in the slow cooker. Make sure the roast is well coated with the marinade.
5. Add the diced fresh pineapple chunks to the slow cooker, distributing them evenly around the roast.
6. Cover the slow cooker and cook on low heat for 6-8 hours or high for 4-5 hours, or until the pork is tender.
7. Optional: Once done, make a cornstarch slurry and pour into slow cooker to further thicken gravy sauce.
