Chicken potpie is both an example of the best of American home cooking and a kind of culinary miracle worker, with a creamy sauce and a flaky pastry lid that are guaranteed to calm even the most jangled nerves. Both the pastry and the filling can be made ahead and refrigerated for up to 8 hours before assembling and baking.
6 Tbs. unsalted butter
1/2 lb. button mushrooms, quartered
1 cup chopped leeks, white and pale green parts
1/2 cup finely diced carrots
1/3 cup fresh or thawed frozen peas
1/3 cup plus 1 Tbs. all-purpose flour
4 1/2 cups chicken stock or broth
1/3 cup dry sherry
2 tsp. minced fresh tarragon
4 cups cooked, shredded chicken
Kosher salt and freshly ground pepper
Double-crust flaky pastry dough
1 large egg
To make the sauce and vegetables, in a large fry pan, melt 1 tablespoon of the butter over medium heat. Add the mushrooms and cook, stirring occasionally, until they begin to brown, about 6 minutes. Stir in the leeks and carrots, cover, and cook, stirring occasionally, until the leeks are tender, about 5 minutes. Remove from the heat and stir in the peas.
In a large saucepan, melt the remaining 5 tablespoons butter over medium-low heat. Whisk in the flour and let bubble gently for 1 minute. Gradually whisk in the stock and sherry and then the tarragon. Bring to a boil, whisking frequently. Stir in the shredded chicken and the mushroom-leek mixture and season with salt and pepper. Let cool until lukewarm, about 1 hour.
Preheat the oven to 400 degrees F. Spoon the chicken mixture into six 1 1/2-cup ovenproof soup crocks or ramekins.
Place the unwrapped dough on a lightly floured work surface and dust the top with flour. (If the dough is chilled hard, let it stand at room temperature for a few minutes until it begins to soften before rolling it out.) Roll it out into a rectangle about 20 by 13 inches and 1/8 inch thick. Using a 6-inch saucer as a template, use a knife to cut out 6 rounds. Beat the egg with a pinch of salt. Lightly brush each round with the egg. Place 1 round, egg side down, over each ramekin, keeping the pastry taut and pressing it around the ramekin edges to adhere. Place the ramekins on a rimmed baking sheet. Lightly brush the tops with the egg. Bake until the pastry is puffed and golden brown, about 25 minutes. Transfer each ramekin to a dinner plate and serve. Serves 6.