In saucepan over medium heat, combine stock, gravy, potatoes, turkey, vegetables, rosemary, sage and 1 cup water; bring to boil. Season with salt and pepper to taste.
Stir in lemon juice and transfer pot pie filling to 9-inch-by-13-inch baking dish; set aside.
In small bowl, whisk egg with 1 tablespoon water; set aside.
On lightly floured surface, lay out puff pastry and cut vent holes with paring knife. Lay pastry on top of filled baking dish.
Place baking dish on rimmed baking sheet and brush pastry with beaten egg. Bake pot pie until pastry is golden brown and internal temperature is 160 degrees F, about 25 minutes. Allow to cool 5–10 minutes before serving.
You can assemble pot pie the morning or afternoon after Thanksgiving, and bake just before guests arrive.