Pat the meat dry and season with salt and fresh cracked pepper.
Heat a heavy bottom pan to high on the stove top. Add a drizzle of olive oil.
Place the meat, fat side up, into the pan and sear for at least 4 minutes to get good caramelization on the surface.
Flip the meat to sear on all sides.
Remove the pork and place in the slow cooker.
Sauté the onion and the apple in the pan until they start to take on color. (optional)
Use some of the liquid from the sauerkraut to deglaze the pan, scraping up the pork bits.
Pour the apples, onions and deglaze into the crock pot along with the rest of the sauerkraut.
Cover and cook on low for 8-12 hours or on high for 5-7 hours.
Remove from the crock pot and place the pork and krout on a serving platter. Allow the meat to rest for 15 minutes before cutting to allow the juices to redistribute throughout the meat.