Go through the lentils and discard any debris or shriveled seeds. Rinse, cover with water and soak for two hours.
Drain the lentils, add fresh water to 1/4-inch above the lentils, bring to a boil and then reduce heat and simmer 20 minutes.
Mix 1 Tbsp. soft butter with 1 Tbsp. flour until it forms a paste and add it to the lentils. Stir the pot until the Beurre Manie has dissolved and the mixture starts to bubble. (Beurre Manie is like a roux, used to thicken.)
Simmer for another 20 minutes or until the lentils are soft but not falling apart. Add the sugar and the salt and vinegar to taste.
The lentils should taste sweet-sour. Serve with fried or pickled eggs.
Note: German cooks prefer the unhulled type of lentils, which require soaking. If all you can find is the hulled variety, you will not need to presoak.