Celery clean, wash, grate on a coarse grater.

Onion peel, chop. Reheat in a saucepan the butter and fry the onion until Golden brown. Add celery, sprinkle with flour and lightly fry all together, stirring constantly (quite a bit, and then everything will turn into a lump)

Pour all the broth (here is a matter of taste, you can try and boil broth and in hot water to dissolve the cube). If you want the soup thinner, you need 1 liter of broth, but I like the cream soup, so I added somewhere in 600 ml. All this bring to a boil and cook for 10 mins on low heat. Season with salt and pepper. Here the main thing - not to overdo, expect that the cheese would add salt!!!

Egg yolk mixed with sour cream, add to soup and bring to a boil, but do NOT BOIL!!!

Remove from heat, season the soup with lemon juice and sprinkle with grated cheese. When cheese is melted, pour into soup bowls and sprinkle with parsley. Everything!! Bon appetit!