Heat butter in a large saucepan over medium heat.  Cook onion, carrot and celery until soft, about 7 minutes.  Sprinkle vegetables with flour and stir to coat. Cook, stirring, for a minute, then add the broth, milk and cream.  Bring to a simmer, add pepper and salt to taste. Add potato, cover, and cook until tender, about 12 minutes.

Remove from heat and stir in frozen peas, leftover turkey and herbs. Let cool for 30 minutes (or see note in post).

Meanwhile, preheat oven to 375ºF with a baking sheet or pizza stone placed on the lowest oven rack.  Roll out bottom pie crust (if using) to about 12" and gently transfer to a 9x2" pie dish. Chill in the freezer for 10 minutes.  When filling has cooled and oven is hot, pour filling into frozen pie crust.  Roll out top crust slightly larger than the pie plate and drape it over the filling, pinching it together with bottom crust. Cut a few slits for steam vents and brush with egg wash. Bake on preheated baking sheet/stone for about 35 minutes, until crust is golden and filling is bubbling out.