Sauté the onion in a little butter,add the stock and cook the vegetables until soft. Remove and keep warm. Add cream and milk, thicken with cornstarch to desired consistency. Season with lemon juice, horseradish,salt and pepper. Bring toa boil and strain the mixture. Add the fish pieces and cook until fish is done (about 2-3 minutes). Serve with boiled potatoes or just root vegetables. Top with shredded horseradish and parsley.