Chilled courgette and rosemary soup

50 g butter
2 medium onions, skinned and chopped
750 g courgettes, sliced
1 garlic clove, chopped
2 tsp fresh rosemary leaves
1.2l chicken stock
salt and freshly ground black pepper
150 ml natural yoghurt to garnish
2 tbsp chopped chives to garnish
Melt the butter in a saucepan and add the chopped onions. saute for about 5 minutes, stirring occasionally, then add the sliced courgettes. cook for a further 5 minutes, then add the garlic, rosemary, chicken stock, salt and pepper. Simmer for about 25 minutes. allow to cool a little. Liquidize and, for an extra smooth soup, sieve the liquidized soup into a bowl. Chill thoroughly in the refrigerator for several hours. Swirl a spoonful of yoghurt into each plateful, sprinkle some chopped chives over the yoghurt and serve.