Clean the leek, cut into fine rings, wash and drain. Melt butter in a pot. Sweat the leek in it. Dust with 20 g flour and sweat it. Deglaze with stock and milk while stirring.
Bring to the boil and simmer for 1 minute. Stir in mustard. Season with salt and pepper. Put aside covered. Cook pasta in boiling salted water according to package instructions. Meanwhile wash the fish, dab dry and cut into 8 slices. Season with salt and pepper. Sprinkle with lemon juice. Put 30 g flour in a deep plate and turn slices in it. Heat the oil in a large pan and fry the slices for 3-4 minutes until golden brown.
Season with salt and pepper. Sprinkle with lemon juice. Put 30 g flour in a deep plate and turn slices in it. Heat the oil in a large pan and fry the slices for 3-4 minutes until golden brown. Serve the pasta, sauce and fish together. Wash the lemon, grate dry and cut into slices. Garnish with lemon slices and serve immediately