Cover potatoes with skin and cook in water for about 20 minutes. Clean, wash and quarter the pointed cabbage and cut into thin strips from the stalk. Peel and finely chop the onion.
Heat the oil in a large pot. Sauté the onion in it. Steam the cabbage briefly. Add 1/8 l water and stock. Bring to the boil, cover and stew for about 10 minutes. In the meantime, preheat the oven (electric cooker: 150°C/circulating air: 125°C/gas: level 1).
Drain the potatoes, rinse and peel them. Peel horseradish, grate finely and mix immediately with lemon juice. Stir mustard and horseradish into the pointed cabbage, except for something to sprinkle on top.
Add the potatoes and stew everything covered for another 5 minutes.
Place the trout fillets on a baking tray lined with baking paper and heat in a hot oven for 3-4 minutes. Season pointed cabbage with salt, pepper and mustard. Serve with the trout fillets and sprinkle the rest of the horseradish on top.