01 -
Heat the oven to 200°C. Rinse sliced leeks thoroughly to remove grit and drain. Melt butter in a wide skillet over medium heat. Cook leeks 7–8 minutes until softened.
02 -
Add chicken and cook 5 minutes, stirring, until the surface is opaque. Sprinkle flour over the mixture and stir for 1 minute.
03 -
Gradually stir in stock, then cream, mustard, salt and pepper. Simmer gently 6–8 minutes until thickened and the largest chicken pieces reach 74°C.
04 -
Transfer to a shallow pie dish about 23 cm across. Let cool for 10 minutes while preparing pastry; the filling should stop steaming strongly.
05 -
Lay puff pastry over the filling, trim to leave a slight overhang and press onto the dish rim. Cut a small steam slit in the center. Brush lightly with beaten egg; discard leftover egg wash.
06 -
Bake 25–30 minutes until the pastry is risen, deeply golden and cooked through and the filling bubbles at the edges. Serve in four portions with a spoonful of filling beneath the crisp pastry.
07 -
Refrigerate cooled leftovers within two hours for up to three days. Reheat covered in the oven until the filling reaches 74°C, uncovering near the end to crisp the pastry.