Peel, wash and slice the potatoes. Heat 2-3 tablespoons of oil in a pan. Fry the potato slices for about 15 minutes until golden brown, season with salt and pepper.
Melt the fat in a pot. Add flour and stir in. Deglaze with milk while stirring, bring to the boil. Stir in broth and mustard. Wash dill, dab dry and chop finely, except for something to garnish.
Stir into the sauce and season with salt and pepper. Peel, halve and slice the onions. Cut the cucumbers into slices. Wash fish, dab dry, cut into cubes and sprinkle with lemon juice.
Season with salt. 5 minutes before the end of the cooking time add cucumber and onions to the potatoes. Heat 1 tablespoon of oil in a pan. Fry the fish for about 5 minutes while turning. Add to the fried potatoes.
Arrange on plates. Add sauce. Garnish with dill.