I find this dish particularly helpful precisely BECAUSE it has quite a few STEPS. The beauty is, you don't have to do it all at once, which is brilliant when you need to do a hundred other things and just have to run out to the post offic. Make the pastry, put it in the fridge and forget about it for a while, make the sauce early in the day and you're two steps ahead, I love anything that makes me feel like I'm two steps ahead.
I use left over roast chicken from a previous meal. Quite often I roast a couple of extra split breasts to use for soup or this pie. At a pinch you could use a rotisserie chicken from the super market.
Pastry
1 1/2 cups of all purpose flour 6 tablespoons of cold, diced, unsalted butter
1/4 cup of cold water with 1 egg yolk beaten in it
1 tablespoon of minced rosemary
Pinch of salt
I'm afraid I am so lazy I only make pastry in the food processor. Pour in the flour, salt and minced herbs and pulse a couple of times. Add the diced butter in batches of 3-4 and pulse a couple of times with each batch to mix them in. Add the cold water and egg yolk with the processor on and continue until it looks like it is coming together. DON'T OVER DO THIS!! stop BEFORE it gets to a ball. Dump it out onto a floured surface and bring together gently using the heal of your hand. Shape into a disk and place onto a sheet of plastic wrap, wrap up and pop in the fridge for at least 30 minutes.

Sauce
4 tablespoons butter 4 tablespoons of flour 1 cup of milk 1/4 cup white wine 3/4 cup of chicken stock bay leaf salt and pepper 1 1/2lbs of cooked chicken Meanwhile, get on with the sauce. In a medium sized saucepan melt 4 tablespoons of butter, add 4 tablespoons of flour and mix. In a measuring jug combine 1 cup of milk, 1/4 white wine and 3/4 organic chicken stock and the zest of a lemon. Gradually add to the butter and flour stirring gently with each addition until all the liquid is absorbed. Add a bay leaf and season with salt and pepper. Let the sauce simmer very gently. Vegetables Olive oil 1 onion, diced 3 sticks of celery, diced 1 leek, finely chopped 3 carrots, peeled and diced 2 cloves of garlic, peeled and chopped In a saute pan add the olive oil, diced onion, celery and carrots. Saute gently for approximately 6-8 minutes until soft but not browned, add the garlic and cook for another few minutes. Skin the chicken and tear into chunks. Add the chicken to the sauce and then add the cooked vegetables. Stir and pour into an ovenproof dish. Roll out the pastry and place on top of the chicken sauce, crimp the edges and cut 4-5 small slits on the top to let the steam out. Beat the egg and egg wash the pastry so it will brown beautifully in the oven. Place the dish onto a baking sheet and pop into the oven for approximately 50 minutes or until crisp and brown. Let the pie cool for 10-15 minutes before serving.