Sauté the onions in the butter, add the Brussels sprouts. Pour in the broth and bring the soup to the boil. Let simmer for 15 minutes. Remove 1/3 of the Brussels sprouts and quarter them. Puree the rest of the Brussels sprouts in the broth. Stir 100 g creme fraiche into the soup, season with salt, pepper and nutmeg. Put Brussels sprouts back into the soup and warm up. Mix 100 g creme fraiche with chopped parsley, season with salt and pepper. Serve the soup with a dollop of parsley cream. You can also use frozen Brussels sprouts.