Wash and peel the potatoes. Trout only from the inside carefully clean, wash and season with salt. Pierce the tail and head of the trout with a needle and kitchen string and pull them together roundly.
Place the trout on the grill of an oven and place over a fat pan. Boil 650 ml water and 1/8 litre vinegar in a pot and pour over the trout. Steam in a preheated oven (electric: 200 °C/ gas: level 3) for 25-30 minutes.
In the meantime, cook the potatoes in salted water for about 20 minutes. Wash 1 orange thoroughly, dab dry and peel in fine strips with a julienne nipper. Squeeze the orange. Heat the fat in a saucepan and sauté the flour in it.
Add stock and milk while stirring, bring to the boil and simmer over low heat for about 5 minutes. Add 50 ml orange juice and season the sauce with horseradish, salt and sugar. Keep warm.
Peel the remaining oranges so that the white skin is completely removed. Cut oranges into thin slices. Clean, wash and drain the salad and pluck into bite-sized pieces. Peel and finely chop the onion.
Mix the remaining vinegar, 2 tablespoons of the remaining orange juice, onions, salt and pepper. Whip the oil into the mixture. Mix oranges, salad and vinaigrette and arrange in small bowls. Wash parsley, dab dry and chop except for a little bit for garnishing.
Loosen the kitchen string from the trout. Arrange fish, sauce and potatoes on plates. Sprinkle with parsley and orange julienne and garnish with parsley.