Peel and chop the celeriac. Cut apples into quarters, remove seeds and dice coarsely. Dice the onions.
Melt butter in a pot and sauté everything over medium heat for 5 minutes.
Add vegetable stock and bring to the boil. Cover and simmer for 30 minutes at medium heat. At the end of the cooking time puree the soup finely with the chopping stick.
Add whipped cream, bring to the boil briefly and season with salt and pepper. Serve with white bread croutons fried in butter.