Polish Recipe: Old-Style Polish Onion Soup

From the book My Homemade Polish Recipes by Ania Zaremba

Ingredients:
11/2 pounds onions
1 leek
2 parsley roots
1/2 cup sour cream
1 tablespoon butter
2 egg yolks
.. salt

Instructions
Rinse, peel, and dice the onions and vegetables. Saute them lightly in butter. Transfer to a saucepan, cover with boiling water, salt, and cook until tender. Cool slightly. Puree in a blender or food processor. Mix the sour cream with the egg yolks and add to the hot soup. Mix well. Salt to taste. Serve with croutons.