Chop the onion, put into a saucepan with 1 tablespoon of margarine and saute. When onion is semi-sauteed add the flour and mix well. Wash the lentils, chop the carrot and put them in the saucepan. Pour the broth and water into the saucepan, add salt as desired and cook the soup for 30 minutes until the lentils are pasty. Strain the soup and pour it back into the same saucepan. Bring to the boil. Meanwhile blend egg yolks with milk well in a bowl and mix this into the soup. Remove saucepan from heat after 2 minutes. The soup is now ready to be served.
Cut 4 slices of white bread into cubes. Fry bread in a pan with 2 tablespoons of margarine. Drain off the margarine and put the bread cubes (croutons) on the soup.