To make the béchamel sauce: Melt the butter in a medium saucepan over medium-high heat until foaming. Add the flour. Cook, stirring, for 1–2 minutes or until bubbling. Remove from the heat. Slowly add the milk, whisking constantly, until the sauce is smooth. Return to high heat. Cook, stirring, for 5 minutes or until the sauce comes to the boil. Remove to a large bowl (see tip).
Meanwhile, melt the butter in a non-stick frying pan over medium heat. Add the leek. Cook, stirring, for 3 minutes or until soft. Increase heat to high. Add the mushrooms. Cook, stirring occasionally, for 4 minutes or until tender. Remove from heat.
Preheat oven to 220°C fan forced. Lightly grease four, 1½-cup capacity ovenproof dishes. Stir the mushroom mixture, feta, chicken and parsley into the béchamel sauce. Season with salt and pepper. Spoon the mixture into the dishes.
Cut the pastry into 6cm squares. Arrange 6 squares, slightly overlapping, over each pie top. Brush lightly with water. Sprinkle with pepper. Place dishes on an oven tray and bake for 25-30 minutes or until pastry is golden and puffed. Serve.