Pick over and wash lentiles, add water to cover and bring to boil. Cover and simmer until the liquid is absorbed. Gradually add the hot meat stock and sieve or put through an electric blender. Melt the butter, add the flour and stir over medium heat for 1-2 minutes, gradually add the soup. Cover and let simmer for 10 minutes. Garnish the soup with croutons fried in some oil.