Cream of Onion Soup

3 Tbsp butter 
1-1/2 cups thinly sliced onions 
1 Tbsp flour 
1/2 tsp salt 
4 cups milk or cream
4 beaten egg yolks 
Salt 
Paprika, freshly grated nutmeg, or Worcestershire sauce (optional) 

Sauté the onions in the butter over medium-low heat until golden brown.
Stir in the flour and salt, then add the liquid of your choice. 
Simmer the mixture, covered, until the onions are very tender. 
Pour a small amount of the soup into the egg yolks and mix, 
then pour that mixture into the soup and stir well. 
Heat a bit more, but do not allow it to come to a boil. 
Season with the paprika, nutmeg, Worcestershire sauce, if desired.