Germans from Russia - Lentil Soup

A great soup from the Germans From Russia experience:

Linsensuppe
(Lentil Soup)

2 cups dried lentils
A meaty ham bone
1 cut chopped celery
½ cup chopped onion
1 tsp sugar
½ tsp dried thyme
1 bay leaf
2 tbls butter
2 tbls flour
Juice of 1 lemon
1 tbls salt
1/2 tsp pepper

Cover lentils with water. Soak overnight. Drain and save liquid. Put lentils into a large kettle. Add enough water to saved liquid to make 9 cups. Add to lentils with the ham bone. Cover and simmer until lentils are tender, two to three hours. Add the celery, onion, sugar, thyme and bay leaf. Cover and simmer half an hour more.

Put soup through a sieve or puree in a blender. Melt butter in saucepan. Blend in flour. Add a little of the soup and mix well. Stir flour mixture into rest of soup. Stir until it boils. Add lemon juice, salt and pepper.

Serve with a sprinkling of scallions or lemon slices on each bowl as garnish