Clean the potatoes and celery and cut into cubes. Peel the onions and cut into small cubes. Sweat in a saucepan with butter and powdered sugar. Deglaze with Noilly Prat and simmer until the liquid has evaporated. Add the prepared potato and celery cubes and pour the hot broth on top. Bring to the boil, then simmer on a low heat for about 25 minutes. Puree finely with a hand blender and season to taste with salt, pepper, nutmeg and chilli salt. Chop the herbs and stir in.