Add all starter ingredients to a clean mason jar. Cover loosely.
Stir once a day, for 5 days. Adding a tsp of already going sourdough starter will kickstart the fermentation process.
You will know that the starter is ready by smell. It should smell like pickles.
Making the soup
In a large soup pot melt the butter. Fry the onions in the butter until golden brown.
Add the 6 cups of water, potatoes, sausage, allspice, bay leaves, vegeta, salt, pepper, and marjoram. You may wish to place the allspice and bay leaves in a sachet (see notes)
Bring the liquid to a gentle boil and cook uncovered until the potatoes are done.
Remove the allspice and bay leaves from the soup. Stir the fermented soup starter and strain it into the soup pot, removing the garlic, bay leaves, and allspice.
Whisk in the sour cream. After whisking, add the hard boiled eggs and stir gently.
Heat on a gentle bubbling simmer until the soup has thickened. This can take between 1 and 2 hours. The flavors will become more potent as you cook and you should expect the soup to reduce at least 1 to 2 inches depending on your pot.
Serve with sourdough bread (recommended).
Notes
It is not pleasant to be enjoying the soup and accidentally chomp on an allspice berry. You can add the allspice berries and bay leaf to a sachet that can easily be removed later.
Keyword Polish, rye, sausage, sourdough,, white barszcz, white borscht