Heat oven to 350 degrees.
Spray a glass 9 inch pie pan with baking spray.
In a large mixing bowl, stir together, chicken, vegetables, mushrooms, sour cream, cream of mushroom soup, poultry seasoning - if using, garlic, onion flakes and 1/2 cup of the shredded cheese.
Pour chicken mixture into prepared pie pan. Set aside.
In a small mixing bowl, combine the first three ingredients of crust mixture. Add water 1 tsp at a time while stirring just until mixture begins to clump together, like pastry dough. (You may not need all the water.)
Pinch bits off dough and sprinkle evenly over the top of the filling, covering the top. Sprinkle with herbs, if using.
Bake for 30 - 40 minutes, until hot and bubbly and crust is golden. (You may wish to cover tent the pie for the last few minutes of baking with foil so the crust doesn't over-brown.