Pat pork tenderloins dry. Season generously all over with salt and pepper.
Sear: Heat neutral oil in a large skillet over medium-high. Sear pork until deeply browned on all sides. Transfer to slow cooker.
Aromatics: In the same skillet, add red onion; sauté 3-4 minutes until translucent. Add garlic, pumpkin pie spice, smoked paprika, and sage; bloom 30 seconds.
Deglaze: Pour in apple cider, scraping up browned bits; reduce 1 minute. Add chicken stock and stir in mustard, brown sugar, soy sauce, and fish sauce.
Transfer: Tip the onion/liquid mixture into the slow cooker. Nestle the seared tenderloins on top.
Cook: Cover and cook on LOW 3-4 hours, aiming for 135-140°F in the center; start checking at 3 hours.
Last hour: Add diced apple around the pork. Whisk pumpkin purée into the cooking juices and turn the slow cooker to HIGH.