Melt the butter in a saucepan, add the leeks, potato, and celery, and cook over gentle heat for 10-15 minutes. Pour the stock over and add most of the cauliflower florets, reserving a few for garnish. Bring the contents of the saucepan to a boil and cook until the vegetables are tender. Allow to cool slightly.
Meanwhile, cook the reserved cauliflower florets in boiling salted water unti