Peel the potato, clean the onion and the fennel bulb, cut everything into 1 cm pieces. Put the broth in a saucepan and cook for 25 minutes.
Put the raw egg in a blender and whisk briefly. Take half of the cooked vegetables out of the broth, add them and puree everything. Put the creamy mass back into the saucepan with the soup, stir and heat, but don`t let it boil anymore. Pour in a little more broth if necessary. Season to taste with salt, pepper and nutmeg.