Prepare the ingredients. Peel and roughly chop the potatoes, onion, celeriac, and carrot. Roughly chop the parsley.
Start the soup. Rinse the soup bones under cold water. Place the bones, vegetables, and potatoes into a large pot. Cover with cold, lightly salted water. Bring to a boil.
Simmer for 45 minutes. Let the soup simmer gently. The bones and vegetables will create a rich, flavorful broth. Skim off any foam during the first 10 minutes.
Remove the bones. Lift out the soup bones. If there is any meat on them, remove it, chop into small pieces, and return to the pot.
Strain or blend. For the classic texture: pass the soup through a fine sieve. For a quicker version: use an immersion blender directly in the pot until smooth.
Season. Add 3 tablespoons of cream. Season generously with salt, pepper, and a pinch of freshly grated nutmeg. Stir well and taste.