Soak the peas in a large bowl of cold water for 2 1/2 hours, drain well in a sieve, rinse well under running cold water, drain again and place in a large saucepan with 1/4 pints (700 ml) water ready to heat to boiling.

While the peas are soaking, coarsely chop the peeled onion and carrot together with the celery, leek and bacon. Sweat all these chopped ingredients in 2 tbsp oil for 15 minutes over a gentle heat, stirring frequently.

Add this mixture to the pre-soaked split peas and water in the saucepan together with the bouquet garni. Bring to the boil, then partially cover so as to allow the steam to escape, reduce the heat to low and simmer for 40 minutes, by which time the peas should be very tender.

Discard the bouquet garni; beat the soup with a hand-held electric beater until it is smooth, or put through a vegetable mill and return to the saucepan. Add the hot stock to this puree until the consistency you prefer is achieved. Season to taste and bring slowly to the boil.

Rinse the chervil or watercress; blanch for a few seconds only in boiling salted water, drain, refresh under running cold water, squeeze out excess moisture and reserve.

Stir the cream into the soup and remove from the heat as soon as it returns to a gentle boil. Add a little more salt if wished, stir in the chervil or watercress and the butter.