Preheat large sauté pan over medium-high heat.
Place potatoes in slow-cooker.
Pat dry pork with paper towel. Season with kosher salt and freshly ground black pepper.
Add avocado oil to sauté pan and sear pork loin on all sides for 3 minutes per side, until golden-brown. Remove pork loin from skillet and place pork loin on top of potatoes in slow-cooker. Keep the skillet on medium-high heat.
Add onions and brussels sprouts to skillet. Sauté for 4 minutes, then place on top of pork loin in slow-cooker.
Pour sauerkraut with liquid into slow cooker. Cover and cook for 6-8 hours on high setting or 12-16 hours on low setting. Remove pork loin from slow-cooker and let it rest for 5 minutes before serving with potatoes, sauerkraut and brussel sprouts.