1. Preheat oven to 425º
2. Melt butter in a pan and add onions and celery. Sauté until tender, about 3-4 minutes
3. Add herbs, salt and pepper and sauté for 1 minute
4. Dust with flour and cook for 3 minutes. Stir continuously so you don't burn the flour
5. Add stock and milk and bring to a boil, let it simmer for 5 minutes
6. Spread mashed potato on the bottom of a casserole dish, add a layer of vegetables, then the turkey and pour the sauce over the top
7. Top with room-temperature puff pastry. Pinch the edges around the side of the dish and cut off extra dough. Brush the dough with the beaten egg yolk and water using a pastry brush and slice a few vents in the top of the pie
8. Place in 425º oven for 15 minutes. The turn oven down to 325º degrees for 20 minutes. Serve nice and warm!

Note - Turkey Pot Pie can be frozen. Just be sure to let all the ingredients come to room temperature before putting in the freezer. Be sure to defrost before baking and add another 5 minutes or so to baking time