Preheat the oven or air fryer to 200°C (fan)
Season the fish with mustard, lemon juice, salt, and black pepper. Return to the fridge while you prepare the pastry
Cut two squares of pastry for the bases and roll out a little more to make slightly bigger
Pile the seasoned fish into the centre of each pastry square
Brush the edges with egg, lay a lattice of pastry strips over the top, then press and crimp the edges with a fork
Trim away any excess pastry and brush the tops with egg wash
Chill the en croûtes in the fridge for 10 minutes to firm up
Sauté the leek, garlic, celery, and shallot in a little oil until softened
Deglaze with white wine (if using) and reduce by half
Stir in the flour, then gradually whisk in the cold stock until smooth
Simmer for 10 minutes until thickened and reduced
Stir through the mustard, cream cheese, and herbs. Season to taste
Bake the en croûtes in the oven or air fryer for 15 minutes until puffed and golden
Spoon the creamy leek sauce onto plates and place the hot fish pie en croûte on top
