Place kiełbasa links in a large pot and add just enough water to cover. Bring water to a low boil and cook for 25 minutes.
Remove the sausage, cut into bite-sized pieces, and set aside. Do not discard the water.
Cut bacon into pieces and fry until crispy. Remove the bacon from the pan and add to a container for future use. (The bacon is not needed for this recipe, only the grease)
Add all but a few tbsp of the grease to the sausage water.
Thinly slice the onions and fry in the pan with the remaining bacon grease until soft. Add to the water.
Drain and rinse the sauerkraut and add to the water.
Add brown sugar and stir to combine.
Add salt & pepper to taste.
Cook on a medium-low heat for 45 minutes.
Add the sausage to the sauerkraut and simmer for 30 minutes.