This recipe is from the cookbook by Richard Bertinet "Pastry"
I halved the filling ingredients for a smaller pie.
Reduce the oven heat to 180ºF.
To make the filling melt the butter in a pan and add the flour, cook for 1 minute, whisking all the time.
Add the milk gradually, whisking until the mixture boils and thickens. Lower the heat to a simmer and let the mixture cook, whisking all the time. About a minute.
Whisk in the egg yolks, cheeses and mustard, and season to taste with the salt and pepper, and nutmeg. Take off the heat and allow to cool.
I like to poach my chicken (skinless breasts or thighs) in a little water with a bay leaf and a few peppercorns. You can fry or grill it if you prefer, but I find the chicken is more tender and white when poached . Shread or chop the chicken up into small pieces.
Fry the button mushrooms gently in the olive oil for a few minutes until soft and slightly golden.(chopped in half if small and chopped into smaller pieces if using bigger mushrooms).
Add the chopped chicken and mushrooms to the sauce together with the chopped French Tarragon and Parsley.
Pour this mixture onto the baked pastry and return to the oven and bake for 20-25 minutes until the filling is golden and just set if you wobble the tray.
Eat while it is warm and runny or serve at room temperature with a green salad.