Heat olive oil in a skillet over medium heat.
Season pork with salt and pepper and place into skillet. Sear on all sides. Then, place into slow cooker.
Add apple cider, apple cider vinegar, sage, apples, carrots, celery, garlic, and onion around the base of slow cooker, stirring to combine.
Cover and cook on low for 8 hours.
Carefully remove pork from slow cooker and place on a platter.
Spoon the apples, carrots, celery, and onion around the pork tenderloin and serve.