Peel and dice the celery and parsnips and cook in lightly salted water until cooked.
Peel and dice the potatoes as well and cook them in salted water separately from the celery and parsnips.
Drain everything and puree together.
Mix the oil with the vegetable stock and heat. Pour over the puree and stir together with the chopped spring onion, sour cream, mustard, salt and pepper.