This pot pie relies on store-bought puff pastry and leftover turkey to give the cook in the family a break after Thanksgiving. But it's not only for the post-holiday lull: use leftover chicken or turkey any time of the year for an easy weeknight dinner.
Filling: Preheat oven to 350 degrees F. In a Dutch oven over low heat, melt the butter. Sprinkle in the flour and cook, whisking, about 1 minute. Whisk in a few tablespoons of stock; when incorporated, continue to slowly whisk in the rest of the stock, a little at a time. (Using small additions of liquid and whisking constantly will prevent lumps.) When all the stock has been added, increase heat to medium-high and bring to a boil.
Meanwhile, remove the rosemary and thyme leaves from their stems by pinching the top of the sprig and pulling straight down. Finely chop the leaves and set aside. Continue boiling the sauce, whisking often, until it's thick enough to coat the back of a wooden spoon, about 2 minutes. Season to taste with salt and pepper.
Reduce heat to medium-low. Add onion, celery, carrots, turkey, thyme, and rosemary. Stir to thoroughly combine, then season to taste with salt and pepper.
Assembly: On a clean work surface dusted with flour, roll out the pastry into a 12-inch square; set aside. Use a slotted spoon to transfer filling to a 9-inch square baking pan. (The slotted spoon allows you to control the amount of liquid that is added to the baking pan.) Lay the pastry evenly over the pan. Fold the excess dough back under itself inside the pan; you can also trim and discard some of the dough as needed. Use a paring knife to make 4 or 5 slits in the top of the pastry to allow steam to escape.
Add an egg yolk to a small bowl, and whisk in water. Brush the egg wash evenly over the top of the puff pastry. Set the baking pan on a rimmed baking sheet to catch any spills, then bake until the crust is golden and the filling is bubbling, 15–20 minutes. Remove from oven and let stand for about 5 minutes before serving.