Fry the onions in the oil in a large covered saucepan for 5 minutes, until the onion becomes soft but not browned. Add the turnips, cover and cook for a further 5 minutes

Add all of the water, bring to the boil and simmer for about 20 minutes, until the turnip is tender.

Liquidise the soup with the watercress, then pour it back into the saucepan and stir in the cream, nutmeg, salt and pepper to taste.