Debark and dice the Brie. Roughly grate the cheddar. Peel and finely dice the onion. Melt butter in a large pot. Sauté onion in it.
Dust with flour and sauté while stirring. Add cider, apple juice and 375 ml water bit by bit while stirring. Stir in stock and bring to the boil. Add cheese bit by bit while stirring and melt in the soup. Season with salt, pepper and nutmeg.
Coarsely chop the nuts and roast them in a pan without fat. Wash parsley, shake dry, pluck leaves and chop finely. Season soup again and serve. Sprinkle with roasted nuts and parsley.