Béchamel Sauce: Mushroom and Leek Pie

1. preheat the oven to 180°C.
2. melt the butter slowly in a saucepan, then turn up the heat and stir in the flour to form a paste.
3. add the milk a bit at a time over quite a high heat, stirring continuously and only adding more milk when the sauce is smooth again.
4. let it bubble for 5 minutes over a very gentle heat, stirring occasionally and season with salt and pepper.
5. put the mushrooms and leeks into the processor with the slicing attachment to shred them up.
6. fry the mushrooms, leeks and thyme in a hot pan for a couple of minutes.
7. put the softened mushrooms, leeks and thyme into the béchamel sauce with the wholegrain mustard, then pour into a deep ovenproof dish.
8. roll out the puff pastry onto a floured surface to the thickness of a coin.
9. cut a thin strip from the pastry.
10. brush over the edges of the dish with the beaten egg and wrap the thin strip around the rim.
11. brush with egg again, then drape the main puff pastry sheet over the top.
12. press the pastry top against the pastry edge and cut away any overhanging pastry with a sharp knife.
13. brush the pastry lid with egg and place in the oven for about 25 minutes until risen and golden.
14. serve with some freshly steamed green vegetables.

serves 4