Wash parsley, shake dry and chop. Clean the leek, wash it and cut it diagonally into rings. Steam leek in hot butter for about 10 minutes. Add 2-3 tablespoons of water if necessary. Season to taste with salt and pepper.
In the meantime, melt butter in a pot for the béchamel. Dust with flour and sauté lightly with a whisk, stirring. Add wine and milk while stirring constantly and bring to the boil. Simmer for about 5 minutes while stirring.
Stir in the mustard. Season with salt, pepper and 1 pinch of sugar.
Rinse fish, dab dry and cut into approx. 12 small slices. Sprinkle with lemon juice. Heat the oil in a pan. Fry the fish for about 2 minutes on each side. Season with salt and pepper.
Do everything. Sprinkle with parsley. Mashed potatoes taste good with it.