Wash the potatoes, dab dry, cut into long slices. Place on a baking tray lined with baking paper. Sprinkle with 1 tablespoon of oil and salt. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 30 minutes.
In the meantime, peel and finely dice the onion for the dip. Dice cucumbers finely as well. Wash the chives and cut into small rolls. Put 2 tablespoons aside for sprinkling. Wash and chop the parsley.
Mix mayonnaise, yoghurt and prepared ingredients. Season to taste with salt, pepper and mustard. Wash the fish and dab dry. Cut into strips of about 6 cm. Sprinkle with some lemon juice, season with salt.
Turn lightly in flour. Heat about 2 tablespoons of oil in a frying pan. Fry the fish for 2-3 minutes while turning. Arrange fish and potato pieces with tartar sauce in portions. Sprinkle with chives and add lemon wedges.