Melt the butter in a medium-sized saucepan, add the chopped vegetables and cook for about 5 minutes, stirring to prevent browning.
Add the lentils and stock, then cover and simmer for 15 - 20 minutes until lentils are tender.
While the lentils cook, make a thin white sauce by melting 15 grams of butter in a large saucepan, stirring in the flour and adding 125 ml of milk.
Bring this mixture to the boil then add another 125 ml of milk, boil again, then add the remaining 250 ml of milk and return to the boil, then remove from the heat.
Puree the lentil and vegetable mixture.
Add the pureed mixture to the white sauce, taste and season if required.
The flavour of this soup will improve as it stands.