Wash the potatoes and cook in boiling water for about 20 minutes. Drain potatoes, quench, peel and let cool. Peel cucumbers, remove seeds. Cut cucumber into cubes.
Peel and chop the onion. Cut the potatoes into slices. Heat 3 tablespoons of oil in a pan. Fry the potatoes until golden brown. Season with salt and pepper and remove. Wash the fish, dab dry, cut into pieces and season with salt and pepper.
Heat 1 tablespoon of oil. Sauté cucumbers and onion in it. Pour in the stock, bring to the boil and cook for 8 minutes. Add cream and mustard. Stir in sauce thickener, bring to the boil and season with salt and pepper.
Heat 2 tablespoons of oil in a frying pan. Fry the fish for about 4 minutes. Add potatoes and fish to the cucumbers and heat everything in it. Wash the dill, shake dry and, except for something to garnish, cut finely and sprinkle over the cucumbers.
Garnish with dill.