Peel and dice celeriac. Peel, quarter, core and roughly dice apples. Also dice the peeled onion. Sweat everything together in butter in a saucepan over medium heat. Pour in the broth and bring to the boil. Cook gently over medium heat for 30 minutes.
At the end of the cooking time, finely puree the soup with the hand blender. Add the cream, bring to the boil briefly and season with salt, pepper and sugar. Decorate with the basil leaves.