Place the onion in a very large saucepan with 7 fl oz (200 ml) of the stock and the wine or vermouth, cover and boil gently over a low heat for 15 minutes. Remove the lid and increase the heat to moderately high to reduce the liquid for 2 - 3 minutes. Draw aside from the heat.

Peel the potatoes and beetroot, slice very thinly and add to the saucepan together with a further 9 fl oz (250 ml) of the stock, the lemon juice, a generous pinch of salt and some freshly ground pepper.

Beat well with a hand-held electric beater until smooth and creamy or put through a vegetable mill. Stir in the yoghurt and a little more salt to taste (the soup will taste less salty when chilled).

Pour into individual soup plates, cover with cling film and place in the refrigerator until just before serving, decorating the well chilled soup with a spoonful of whipped cream and some chopped chives.