Wash celeriac well, trim root and leaf base. Place celery on a cutting surface and trim the skin from top to bottom.
Cut the celery into quarters. Using a kitchen knife in a food processor, cut into fine strips and blanch the celeriac strips in a little salted water for about 1-2 minutes.
Drain and rinse with cold water. Drain and let cool.
Peel carrots and also cut into very fine sticks.
Remove two-thirds of the green and the root end from the spring onions, clean and cut into fine rings.
Combine celery, carrots and green onion in a large bowl.
Whisk together salt, vinegar, pepper and parsley. Pour in oil while whisking throughout. Mix in sour cream.
Pour vinaigrette over prepared ingredients; mix well. Season to taste.
Cover and let celery salad stand for about 2 hours. Season again to taste.