Peel and dice potatoes, carrots, celery and celeriac in 5mm cubes. In a sauce pan over medium heat add a little olive oil and fry diced vegetable until lightly brown. Stir in the vegetable stock and let it cook for 10 min. Add sliced frankfurters and crushed vermicelli. Mix egg yolk, yogurt and flour in a small sauce pan. Add little veg. stock to warm it up and let it cook stirring continiously. Cook it for 2-3min. and stir in the soup. Add frozen peas and cook for another 2-3 min. Serve it with chopped parsley.