Start by making the pastry dough. Mix flour, salt and shortening in mixer.
Using a pastry hook or mixer blend on lowest speed until you have a coarse mixture.
Add in ice water 1 tablespoon at a time, allowing all water to be incorporated into flour. Scraping the sides of mixing bowl so that all the flour gets mixed in.
Keep mixing on low sped until the dough starts to pull away off the sides of the bowl and forms a ball. Dough should feel soft, smooth and tacky but not sticky.
Roll dough into a ball and cover with plastic wrap. Refrigerate for 30 minutes.
Melt butter in a large skillet and add onion , celery and thyme and saute until onion is translucent.
Add flour. Flour will stick to onions and celery and will become very thick.
Add chicken stock and simmer until bubbly.
Add remaining ingredients and add salt and pepper to taste.
Place filling in large casserole dish
Preheat oven to 425 degrees Fahrenheit
Remove pastry from refrigerator, unwrap from plastic and place on a floured surface.
Dust your rolling pin with flour, and begin to roll out dough into a rectangle. Make large enough to fit over entire casserole dish.
( You may want to fold edges in to get the perfect rectangular shape).
To transfer crust, fold in half and then carefully place onto half of the casserole over filling and then unfold other half over remaining filling.
Carefully presses all edges with a large tined fork.
Using a sharp knife, cut into the pastry making small slits to allow air out while baking. ( I like to make a chicken design into the pastry, although most times, it does't resemble a chicken at all).
For and extra special touch. Beat 1 egg white until foamy and brush on over entire pastry.
Bake for 40 minutes or until golden and bubbly.