I blanched the watercress in salted boiling water for a minute and shocked in ice water. This ensures that the cress keeps it's bright green color. I diced some celery root (potato can be used too) and cooked it in chicken stock. I then strained it, reserving the stock. I sauteed the celery root and watercress with some onions for a few minutes.
The celery root, cress and onions are then pureed with some of the reserved stock, milk and sour cream and strained. The soup should have a similar thickness to heavy cream. I garnished it with some diced celery root and sauteed red onions.