Polish sour rye soup

For the kwas:
75 g wholemeal rye flour
600 ml boiled, cooled water
1/4 clove garlic
For the soup:
1.25 litres stock from vegetables or beef/chicken bones
100 g bacon
100 g onion
1 can of mushrooms
400 ml kwas
300 ml sour cream
5 medium potatoes, cooked and diced
100 g smoked sausage, diced

1. Rinse out an earthenware jar or any non-alluminium container with boiling water. (Note: the alluminium would react with the acidity of the kwas). Put the flour in the jar and mix to a liquid paste with a little of water. Leave the mixture to settle for a few minutes, and then pour on the remaining boiled water. Chop the garlic and add. Cover the top of the jar with muslin or pierced cling film and leave in a warm place for 4 to 5 days to ferment. Strain and use as required. If stored in an airtight container, it will keep for a few weeks.
2. Heat the stock. Chop bacon and onion and add to stock. Simmer for 10 minutes. Add mushrooms, kwas, cream and garlic. Season with salt and pepper. Allow to simmer for 20 minutes and then add potatoes and sausage. Bring to boil. Serves 6.