Start by making the rye sour, about 4 days before you make the soup. Mix the rye flour and blood-temperature water in a large bowl. Cover with a tea towel and set aside to ferment in a warm place (an airing cupboard is good) for 4-5 days.
Once the rye sour is ready, make the soup. Place the onion, carrots, celeriac, and leeks in a large saucepan and cover with 2 litres water. Bring to the boil and simmer for about 30 minutes. Add the sausage and cook for 20 minutes more. Set aside to cool then strain through a sieve into a new pan. Discard the vegetables, but reserve the sausage and slice thickly.
Skim any fat off the stock and add the fermented rye sour and the potatoes. Bring to the boil and simmer for about 15 minutes, until the potatoes are just cooked.
Make a paste by mixing the 20g plain flour with a little water. Briskly whisk this into the soup with the crushed garlic, the oregano and plenty of seasoning. Simmer for another 5-10 minutes, stirring often, until the potatoes are very tender and the soup is thickened. Add the sausage and warm through.
Meanwhile, cover the eggs with plenty of cool water in a small saucepan and bring to the boil. Simmer for 8 minutes then run briefly under cool water, peel and halve.
Serve each bowl of soup topped with a halved egg. Accompany with rye bread.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.