To make the bechamel, melt the butter in a medium sized saucepan, add the flour and stir vigorously. It’ll go all lumpy and start to resemble polenta (this is called a roux).
Cook over a gentle heat for two minutes – try not to let the saucepan burn. If the roux is starting to brown, remove from the heat and keep stirring.
It is really important to cook the roux for this period of time, otherwise the béchamel will taste of uncooked flour. Add a good splash of milk, and mix vigorously with a whisk.
It will go all lumpy, but just keep whisking. Add more milk after each bit becomes incorporated and keep heating gently. Gradually add all the milk, and cook for a further 5 minutes, stirring gently, ensuring the bottom of the pan is not burning.
Season and set aside. You can do this at least 24 hours in advance.
Heat up one tablespoon of olive oil in a saucepan and cook the chicken pieces over a high heat until they start to colour. Season, and add half the butter.
Cook for a further 5 minutes over a medium heat. When fully cooked, transfer to the casserole dish, spreading the chicken pieces out evenly on the base. Put the saucepan back on the heat and add the remaining tablespoon olive oil.
Fry the leek and mushrooms. Season, and when the leeks are just starting to brown, add the butter. Pour on top of the chicken.
Top with crumbled blue cheese, and pour over the béchamel.
If you have made the béchamel in advance, heat it up in a saucepan before pouring it over the chicken and leek mixture. Top with the rolled out piece of puff pastry and cut off any excess.
You can allow the pie to cool fully, and then refrigerate, or you can cook it straight away at 180 C for 35 minutes.
During cooking, the pastry may rise up. If this happens, simply stab it with a knife to let the air out. If you think the pie is getting too golden brown, cover it with tin foil. When the pastry is golden brown and the filling piping hot, it is ready to serve.