Wash the fish, pat dry, cut into pieces and sprinkle with lemon juice. Cut the cucumbers into slices. Clean and wash spring onions and cut them into pieces. Peel, wash and thinly slice the potatoes.
Heat four tablespoons of oil in a pan, add potato slices and season with salt and pepper. Fry over medium heat, turning several times for 15 minutes. Add spring onions five minutes before the end of the frying time.
Heat the remaining oil in a second pan. Fry the fish until golden brown and season with salt and pepper. Fold the fish and cucumber slices into the potatoes. Season to taste with salt and pepper. Heat the fat in a saucepan.
Sweat flour in it, deglaze with broth and milk. Stir in mustard and let everything simmer for five minutes. Season to taste with salt and pepper. Wash the dill, dab dry, chop finely and add to the sauce.
Pour sauce over potatoes and fish.