Wash the lentils and soak them overnight. In the morning cook them in fresh water until soft - about 2 hours. Then rub them through a sieve.
Cook the butter and flour in a saucepan until brown. Add the vegetables and cook for 15 minutes; then add the bouillon, and cook for half an hour; then put in the lentils and rub again through a sieve. Let this whole mixture now boil for about 10 minutes.