Step 1

Melt the butter in a saucepan. Add the flour and whisk for a few minutes to cook the flour.

Step 2

Add the stock gradually, whisking constantly to avoid lumps, then add the salt and cream. Cook for a few minutes, stirring constantly until thickened. Then take it off the heat and leave to cool.

Step 3

Put the buttermilk, melted butter, eggs and salt into a large mixing bowl and whisk together. Add the flour and mix well, then turn out onto the worktop and knead for a few minutes before wrapping in cling film and putting it in the fridge for at least 30 minutes.

Step 4

Prepare the vegetables: Saute the onion and celery in one tablespoon of oil, until soft and golden brown. Switch off the heat and stir in the peas. Boil the carrots until al dente, then drain and mix with the other vegetables. Add a few tablespoons of bechamel to the vegetables - just enough to bind them together, then leave to cool completely.

Step 5

Mix the chicken and mushrooms. Add the remaining bechamel and mix well, then leave to cool completely.

Step 6

Assemble the pie: Cut off one-third of the pastry and roll it out to a 10 inch circle. Lay it on a flat baking sheet. Put 2/3 of the chicken mixture into the middle of the pastry and shape into a flat circle, leaving 5 centimeters around the edge. Then add 2/3 of the vegetable layer, flatten the top and smooth the edges. Repeat one more layer of each flavor, ending up with a tall, smooth mound.

Step 7

Roll the remaining pastry out into a 15 inch circle, and cover the pie fillings with it. Seal the pastry together with your fingers around the bottom, then trim so there is just a 1 centimeter lip all around. Press around this lip with a fork, then carefully tuck it underneath the pie, using a small palette knife to help. Cut a 3 centimeter steam hole in the top of the pie, then mIx the egg and water together then glaze the whole pie.

Step 8

Roll out the excess pastry and cut out leaves to decorate. Create a small plait to encircle the steam hole. Stick the pastry leaves around the bottom edge of the pie and secure the plait on top around the steam hole. Put the pie into the fridge to chill for one hour.

Step 9

Preheat the oven to 400F. Cook the pie for 40 minutes, turning it around halfway through to help it brown evenly. Then let it rest for 15 minutes before serving.