Strain stock. You should end up with 4 to 6 cups. If you want to reduce stock further (if you have at least 6 cups or you want a little more flavor) just slow simmer while your working on other steps.
for the Filling
Saute onion and celery in butter, until onions are transparent.Don't brown.
Add sliced mushrooms and continue to cook until mushrooms loose their moisture. Add sherry, bring to simmer for about a minute, then remove pan from heat.
Make slurry out of water and flour.
In a pot large enough to accommodate all filling ingredients, heat stock while whisking in slurry. Bring to a simmer until stock starts to thicken.
Add sauteed ingredients to simmering sauce along with chicken base
Add bay leaf and allow to cook for about ½ hour.
Add chopped carrots & pulled chicken meat, dried herbs. Allow to cook until carrots are just tender. Add peas, corn & mashed potatoes.
Taste & adjust seasoning with salt & white pepper to your liking.
for the Pate Brisee
Cut chilled butter into small pieces and add to flour & salt. With fork or pastry blender, work ingredients together until mixture looks like course meal. I usually use my hands at the end. Add enough water so that the mixture just sticks together.
Make a disk and wrap in film and refrigerate at least 1 hr or up to 3 days.
Once dough has rested, remove from fridge, remove wrap and let set on rolling surface for 20 minutes.
While your dough is waiting, preheat oven to 425* and choose your potpie dish. I think a depth of 3" is about perfect.
Place filling in dish. Leave about ½" clearance on top.
Lightly flour rolling pin & rolling surface. slowly and gently begin to roll disk. Have a little patience here. When I first starting making this, I would work my pin too fast and too hard. This led to split sides and a mass of dough that was impossible to remove from surface in one piece. Flip back and forth, gently sprinkling with flour prevent the dough from sticking to the surface.
Once the dough is approximately ⅛" thick, it's time to place atop the chicken mixture.
Roll your dough over pin and gently cover the surface of your soon to be pot pie. If you have trouble with this move, use a cereal box side that you've cut apart, and wiggle under dough as you would a peel for pizza.
Pinch dough around sides, venting crust out. Cut vents in crust. I like an S shape with a few small slits.