Melt the butter in a large saucepan and add the vegetables. Cover the pan and stew gently for 10 to 15 minutes, until vegetables begin to soften. Meanwhile, thinly pare the lemons using a potato peeler. Blanch the rinds in boiling water. Add the rinds and juice, stock and bay leaves to the saucepan. Season with salt and pepper and bring to the boil. Cover and simmer for 40 minutes or until both celery and carrots are very tender. Cool the soup a little, remove the bay leaves, then puree the pan contents in a blender until smooth. Return the soup to a clean pan and reheat gently, stirring in the cream. Do not boil. Adjust the seasoning to taste. Garnish with chopped spring onions or chives and lemon slices. May be served hot or chilled.