Heat the oil in a large saucepan and add the onions. Cover and cook over a very low heat for about 20 minutes until very soft but not coloured stir occasionally

Add the chicken stock to the saucepan and bring the mixture to the boil

Meanwhile great the courgettes and stir into the boiling stock with the pastor.

Reduce the heat and simmer for 15 minutes or until the pasta is tender

Season to taste with lemon juice salt and pepper

Stir in the chopped fresh chervil and add a swirl of the sour cream before serving