Remove the pork loin from the refrigerator to bring it to room temperature, which ensures even cooking throughout the roast. While it warms, peel and core the apple, cutting it into 1-inch chunks, then dice the onion into ½-inch pieces. Pat the pork dry with paper towels, then season it generously on all sides with the salt and cracked black pepper, making sure to coat every surface. This seasoning step is crucial for developing flavor throughout the meat as it cooks slowly.

Distribute half of the prepared apples and onions evenly across the bottom of your slow cooker, creating a flavorful bed that will infuse the cooking liquid. Place the seasoned pork loin on top of this base. Pour the sauerkraut with its liquid over the pork, then sprinkle the brown sugar evenly across the sauerkraut—the sugar balances the sauerkraut's acidity and complements the apple's natural sweetness. Arrange the remaining apples and onions around the pork, then pour in the apple juice. Distribute the butter pats over the top of everything. I like to nestle the butter pieces so they melt gradually into the liquid rather than all at once, creating a richer sauce.

Cover the slow cooker and set it to Low for 7-8 hours, or High for 4-5 hours if you're short on time. The pork will become increasingly tender as the collagen breaks down and the meat absorbs the apple, sauerkraut, and butter flavors. I prefer the Low setting because it produces more tender, evenly cooked meat and allows the flavors to develop more fully, but either setting will give you delicious results. The pork is done when it shreds easily with a fork and reaches an internal temperature of 145°F.

Carefully remove the pork loin from the slow cooker and place it on a cutting board. Using two forks, shred the pork into bite-sized pieces, discarding any excess fat if desired. Return the shredded pork to the slow cooker and gently mix it with the apples, onions, sauerkraut, and all the cooking liquid until well combined. The pork will break apart easily at this point, and mixing it back into the sauce allows all the flavors to meld together beautifully.

Give the mixture a taste and adjust the seasoning as needed with additional salt and pepper. The balance of sweet apples, tangy sauerkraut, and savory pork should be harmonious. Serve warm directly from the slow cooker into bowls or plates.