German potato soup

7 slices lean bacon
3 large onions, finely chopped
4 1/2 Tablespoons flour
9 cups beef broth
6 large potatoes, peeled and sliced thinly
3 egg yolks
1 1/2 cups sour cream

Dice the bacon and saute it in a deep kettle for 5 minutes.  Stir in chopped onions and saute until soft (approx. five minutes).  Blend in flour and gradually add broth.  Add sliced potatoes to simmering broth and cook for 1 hour.  Beat the egg yolks with the sour cream.  Stir 1/2 cup of hot soup into the egg yolk mixture and add this to the soup.  Simmer ten minutes more, stirring constantly.  Serve immediately.