Cook the pasta according to the instructions on the packet.
Cut the fish into 8 pieces. Season with salt and pepper and drizzle with a little lemon juice. Put 30 g flour in a deep plate and roll the fish in it.
Heat the oil and fry the fish in it for 3 - 4 minutes.
Cut the leek into fine rings and sauté in the butter. Dust with the remaining flour and sweat. Pour in the milk and stock while stirring. Bring to the boil and simmer for approx. 1 min. Stir in the mustard. Season to taste with salt and pepper.