Mix the olive oil, rosemary, thyme, garlic, salt, and black pepper in a small bowl to form a paste. Rub the paste evenly over the entire pork loin.
Place the sliced apples and onions in the bottom of the slow cooker to create a bed. Set the herb-crusted pork loin on top of the fruit and onions.
Pour the apple cider around the pork without disturbing the herb rub. Cover the slow cooker.
Cook on low for 6 to 7 hours until the pork reaches an internal temperature of 145°F. Remove the pork to a cutting board and let it rest for 10 minutes.
Slice the pork and serve it with the apples, onions, and cooking juices from the slow cooker. Sprinkle the fresh parsley over the top before serving.