Clean, wash and cut the leek into rings. Heat 2 tablespoons of butter and sauté the leeks for about 10 minutes, turning them over. Add 2-3 tbsp. water if necessary. Season to taste with salt and pepper.
Heat the butter in a saucepan over medium heat. Dust with flour and sauté lightly while stirring.
Stir in the milk and Riesling bit by bit with a whisk. Bring to the boil while stirring and simmer for about 5 minutes, so that the flour taste evaporates.
Finally, refine the sauce with mustard and season to taste with salt, pepper and 1 pinch of sugar. Keep warm.
Wash salmon fillet, dab dry, cut into 12 small slices. Sprinkle with lemon juice. Heat the oil in a large pan and fry the salmon on each side for about 2 minutes. Season with salt and pepper. Wash parsley, cut leaves into fine strips and stir into the leek. Arrange everything. Mashed potatoes taste good with it.