Wash the potatoes well and cook covered for about 20 minutes.
Clean the salad, wash and shake dry. Peel and finely chop the onion. Mix apple juice, vinegar, salt and pepper. Fold in 2 tablespoons of oil. Stir in half onion cubes. Mix salad and marinade.
Wash the fish and dab dry. Sprinkle with lemon juice and let it steep a little. Heat butter. Sweat the flour lightly in it. 1⁄4 l Stir in water and milk. Bring to the boil and simmer while stirring for about 5 minutes.
Refine the sauce with crème légère and season to taste. Keep warm.
Heat 2 tablespoons of oil in a coated frying pan. Fry the fish for 3-4 minutes on each side. Season with salt and pepper.
Wash the dill, shake dry and cut finely. Drain the potatoes. Quench as desired and remove the skin. Heat with dill in the sauce. Serve everything. Sprinkle with remaining onion cubes.
Garnish with lemon.