Wash the potatoes thoroughly and cut into slices. Spread on a baking tray lined with baking paper, season evenly with salt and pepper and sprinkle with 2 tablespoons of oil. Cook in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 30-40 minutes.
Peel the onion and cut into strips. Wash and clean the cucumber and slice it into thin slices. Heat 1 tablespoon of oil. Roast the mustard in it while stirring. Add onion and fry for about 2 minutes until transparent.
Add the cucumber after about 1 minute. Deglaze with vinegar and season with sugar and salt. Wash coriander, shake dry. Pluck the leaves from the stems, except for a little bit for garnishing, and cut them into small pieces.
Stir under the chutney and set aside. Wash the clods thoroughly on the outside and inside, season with salt and turn in flour. Heat 2 tablespoons of oil in 2 large pans. Fry the white side of the plaice for 3-4 minutes until crispy brown.
Carefully turn the plaice with a flat and as wide a spatula as possible and fry for another 3-4 minutes. Arrange plaice and chutney on plates and garnish with coriander. Add potatoes.