For easy clean-up, place a cooking liner in your slow cooker.
Cook bacon in fry pan over medium heat. When cooked, place bacon on paper towel to drain. (Do not remove bacon grease from pan.) When cool, crumble bacon and set aside. Add bratwurst to hot bacon grease and do a quick browning.
In the slow cooker, combine the sauerkraut, apples, bacon, brown sugar, onion and mustard. Place bratwurst on top of sauerkraut mixture.
Cover and cook on low until a thermometer inserted in the sausage reads 160°F, 4-6 hours.
Place brats in buns; using a slotted spoon, top with sauerkraut mixture.