In a large saucepan over medium high heat, saute the bacon for about 7 minutes, or until lightly browned. Reduce heat to medium, add the butter or margarine and onions and saute for about 5 minutes, or until onions are tender. Add the flour and stir well for one minute.
Remove saucepan from heat and gradually add the water, stirring well. Return to stovetop, raising heat to high. Add the bouillon and potatoes, bring to a boil and then reduce heat to low. Simmer uncovered for about 10 minutes, or until potatoes are tender.
In a small bowl, combine the egg yolks and the sour cream and mix together. Gradually add this mixture to the soup, stirring well. Heat through, but do not boil. Stir in the parsley just before serving.