The main ingredient is sauerkraut - cabbage that has been finely chopped and 'pickled' by leaving it in salt for a few weeks. The salt makes the cabbage tangy and slightly fizzy. Many Slovak families used to make their own sauerkraut every autumn, in big wooden barrels, to last all winter.
Slow-cooking the soup is the secret. The sauerkraut goes in first, with sausage, smoky meat, dried mushrooms and onions. It simmers gently for an hour or more, sometimes with paprika for warmth and a spoonful of sour cream stirred in at the end. The longer it cooks, the richer it tastes.
Kapustnica is the traditional soup for Christmas Eve dinner (Štedrý večer) in Slovakia. After the family has lit candles around the table, the meal usually begins with a steaming bowl of kapustnica. Some regions add little prunes for sweetness, others add wild mushrooms gathered from the forest in autumn.
Because every family has its own recipe, there is friendly arguing across Slovakia about whose kapustnica is best. Grandmas guard their personal versions carefully. Children grow up knowing exactly how their grandmother makes it - and they often can't quite get it right themselves until they have tried many times.