Cut leeks lengthwise, wash and cut into 1 cm thick rings.
Wash and peel vegetable and cut into equal-width strips.
Preheat the oven to 60 ° C.
Heat the soup and wine with a little caraway, add and boil vegetables. Add fillets and simmer for 20 minutes at low heat. Cover pot except for a small gap.
Mix cornstarch until smooth in a little cold water. Remove fillets from the soup, wrap in aluminum foil and keep warm in oven. Boil soup and thicken with cornstarch. Season with salt and pepper. Cut fillets into slices and serve with grated horseradish.