Cut the cauliflower into small florets.
Heat up the oil in a pot.
Add the chopped carrots and onions, and fry for a few minutes, until the onion becomes translucid.
Add the chopped potatoes and mix for a minute or so.
Then, fill the pot with water and bring it to boil, slowly.
After around 20 minutes, add the cauliflower florets.
The cauliflower takes 10-15 minutes to become tender.
Meanwhile, make the dressing for the soup.
Mix the egg yolk with the sour cream in a big bowl.
Start adding hot soup to the sour cream, slowly, mixing continuously.
Continue until the temperature of the sour cream liquid reaches the one of the soup.
Pour the dressing into the pot, mixing it well.
Add the vinegar, one tablespoon at a time, and taste after each one until you reach the desired sourness.
Serve hot, alongside more vinegar on the side.