Rinse and peel the potatoes, peel the onions and cut both roughly into cubes, fry briefly in the fat on 3 or automatic heat 12. Add clear soup and cook for 25 minutes on 1 or automatic heat 5 to 6.
Add whipped cream and horseradish 5 minutes before end of cooking time. Blend with a hand blender, season with salt, pepper and vinegar. Serve with cress cut from the bed and smoked trout cut into strips.