1 Roast the chicken: Preheat the oven to 400°F. Arrange the chicken pieces on a sheet pan or baking tray and coat them lightly with olive oil. Sprinkle both sides of the chicken pieces with salt and pepper.
Place the tray of chicken in the oven and roast for 30 to 35 minutes or until the internal temperature of each piece reaches 160°F to 165°F.
2 Cool, then shred the chicken: Remove the roasted chicken from the oven and allow the meat to cool for 15 minutes at room temperature. Leave the oven on as you’ll need it to bake the assembled pot pie.
Once the chicken is cool enough to handle, remove the skin and shred the meat from the bones using your hands or two forks. Don’t shred the chicken too finely, instead leave nice chunks of meat to create that rustic pot pie filling.
Set the chicken aside while you prepare the rest of the filling. Discard the skin and bones or save them to make bone broth or stock.
3 Sauté the aromatics and make the roux: In a Dutch oven over medium-high heat, melt the butter. Once the butter has melted and begins sizzling, add the white onion, celery, and garlic. Using a large spoon to stir, sauté the vegetables for 3 to 4 minutes or until they become translucent.
Reduce the heat to medium and sprinkle the flour over the veggies and fat in the pot. Stir the flour into the vegetables to create a roux, then cook the mixture for one minute to remove the raw, floury taste. The roux should look pale yellow and leave a thick coating on the vegetables.
4 Make the gravy: In a small bowl or large liquid measuring cup combine the chicken stock and milk. Continuously stir the roux coated vegetables while adding the chicken stock-milk mixture to the vegetables one cup at a time. Once all of the liquid has been added, simmer the gravy until it thickens, about 5 to 6 minutes. The gravy will go from a thick, paste-like consistency to a smooth gravy that coats the back of your spoon.
5 Fold in the shredded chicken and the frozen, thawed vegetables: When the gravy has thickened, fold in the chicken you shredded earlier, the peas and carrots, as well as the rosemary, thyme, and sage. Stir to completely coat the chicken and veg in the gravy.
Taste the mixture and, if needed, adjust the seasoning with the salt and pepper. Pour the filling into a 9 x13-inch baking dish and allow it to cool for 15 minutes.
6 Roll out the puff pastry dough and make the egg wash: On a lightly floured surface use a rolling pin to roll out the sheet of puff pastry into an 11 x 15-inch rectangle that’s 1/8-inch thick. In a small bowl, whisk together the egg yolk and water to make an egg wash.
Lay the rolled sheet of puff pastry on top of the baking dish. Brush the surface of the pastry with the egg wash and fold any excess dough over the top of the dough to create a crust. Crimp the edges as you would pie dough, then brush the fluted edge with more egg wash. Sprinkle a pinch of salt over the surface of the dough, then use a knife to cut small slits in the surface to act as vents.
7 Bake the pot pie: Place the baking dish on a larger sheet pan to catch any spills in case the filling bubbles over the edge of the dish. Bake the pot pie for 20-25 minutes or until the top is golden brown and the filling gently bubbles up through the vents.
8 Cool slightly, then serve: After baking, remove the chicken pot pie from the oven and allow it to cool for 15 minutes prior to serving. The filling will be very hot, so take care when eating.