- Place sauerkraut and liquid in a pot filled with just enough water to cover the sauerkraut. Bring to a boil, then remove from stove and transfer to a colander.
- Drain and rinse the sauerkraut well with cold water to prevent further cooking. With your hands, squeeze as much liquid from the kraut as you can to prevent the filling from being too soggy.
- Place the sauerkraut on a large cutting board and chop fine with a large, flat-bladed knife.
- Fry the onion until it is translucent, then add the drained sauerkraut. Season with salt and pepper to taste.
- Simmer for 15 minutes and let it cool before using. For best results, refrigerate a few hours before using to fill pierogi.
Notes, tips, and variations
- The sauerkraut boiling step may be omitted if you like really strong sauerkraut.