In a large saucepan or Dutch Oven, combine the sauerkraut, diced apple, caraway seeds (if you're using them), and brown sugar. Bring to a simmer over medium heat, then reduce heat to medium-low, cover, and simmer for 90 minutes-2 hours, stirring occasionally.

When the sauerkraut is almost done simmering, preheat oven and place the bacon on a foil-lined baking sheet or in a large cast iron pan and bake for 20-25 minutes or until crisp. Remove from oven, drain the bacon on paper towels, and crumble the bacon. Set aside.

If you used a baking sheet, transfer 2 tablespoons of the bacon drippings to a frying pan. If you used a cast iron pan, drain all but about 2 tablespoons of bacon drippings and return the pan to the stove. Heat the drippings over medium heat. When hot, add the sliced onions and kielbasa. Cook for 10-15 minutes, stirring frequently, until the onions are caramelized and the sausage is crisping up on the edges. Transfer the onions, sausage, crumbled bacon, and sauerkraut mixture to a slow cooker and cook on high for 2-3 hours or on low for 4-6 hours. Serve with German potato salad or roasted potatoes.