You can use different vegetables for this soup depending on the season. This is a thick, winter version, but a spring vegetable soup is also delicious.
Heat the butter in a saucepan, add the vegetables, cover, and cook over a low heat until soft. Add the bouquet garni. Pour on 1.5litres water, bring to the boil, reduce the heat and simmer for 30 minutes. Lift out and discard the bouquet garni.