Melt the butter in a large saucepan. Add the finely chopped onion and sauté gently for about 5 minutes until the onion is soft but not brown.
Trim and slice the courgettes, add to the pan and mix with the onion and butter. Stir in the chicken stock and bring up to the boil. Cover and simmer gently for 30 minutes. Draw off the heat and purée the liquid and vegetables from the pan in a blender. Return to the pan, check seasoning, reheat and serve.