Wash potatoes well, dab dry and cut into slices. Put them on a baking tray lined with baking paper. Sprinkle with a little oil. Season with salt and pepper. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for 45-50 minutes.
Peel and finely chop the onion. Finely dice the cucumber. Wash herbs and shake dry. Cut chives into fine rolls, chop parsley. Mix everything with salad cream, yoghurt and mustard.
Season to taste with salt and pepper.
Wash the fish, dab dry and cut into bite-sized pieces. Sprinkle with lemon juice and season with salt. Turn the fish in flour.
Heat 2 tablespoons of oil in a coated frying pan. Fry the fish for 4-5 minutes while turning. Arrange fish and potato pieces in portions with tartar sauce.