Wash the leek and cut into fine rings. Heat the butter in the pot and sauté the leek briefly. Dust the leek with 2 tablespoons of flour and sauté. Pour in 400ml water and milk while stirring. Stir in vegetable stock. Bring everything to the boil and simmer for about 1 minute.
Stir in mustard. Season to taste with salt and pepper.
Cooking pasta.
In the meantime, cut the lemon in half and squeeze out 1 half. Wash fish, pat dry and cut into 8 pieces. Season with salt and pepper. Drizzle with lemon juice.
Put 3 tablespoons of flour in a deep plate and turn the pieces of fish in it. Heat the oil and fry the fish until golden brown. Cut the remaining lemon half into slices for garnish.