Core the apples, do not peel them and add them to the liquid in large cubes. Also add the leek, cleaned and roughly chopped. Peel the potatoes and add cubes to the apples and leeks.
Bring to the boil once, then simmer covered for 15 minutes over a low heat. Turn off the heat and pull the pot off the stove.
Stir in horseradish and cream, season with salt and pepper. Add the cold butter and puree everything to the desired consistency using the blender. Arrange the soup in plates, garnish and serve.