In a large saucepan, melt 1/4 cup (60 ml) butter over low heat. Add the leeks and green onions and cook, stirring occasionally, for about 5 minutes or until softened (do not brown). Add the potatoes, water, salt and pepper and bring to a boil. Reduce heat, cover and simmer for about 30 minutes or until potatoes are tender. Allow to cool slightly.
Using a food processor or blender, reduce the preparation to a puree, several times as needed. Pour the leek puree into the pan and bring to a boil. Add the warm milk and the rest of the butter and mix. Heat over low heat (do not boil).
NOTES:
When serving, garnish each serving with croutons.