Preheat oven to 425 degrees.: When the oven reaches 425 degrees , you will notice a dry, hot air that crisps pastry quickly, which helps the top crust set and brown without overcooking the filling. Preheating ensures the pie goes into a stable environment so the crust begins to puff and seal, creating flaky layers. A common oversight is not preheating long enough, which leads to a soggy bottom, so wait until the oven signals it is fully up to temperature. The sound of the preheated oven cycling is a subtle cue that it is ready.
Place potatoes and carrots in a small saucepan, cover with water, and bring to a boil for 5-8 minutes to soften. Drain.: As the water comes to a rolling boil, you will see bubbles rising and the scent of starch and sweet carrot releasing into the steam, signaling the vegetables are loosening. Cook the diced potato and sliced carrot until a fork slides through with slight resistance, then drain thoroughly to avoid diluting the sauce. This partial cook guarantees tender vegetables after baking, and undercooking here leaves hard bites in the finished pie. Rinsing briefly in hot water after draining is unnecessary and can cool them too fast.
Melt 1/2 cup butter in a large sauté pan. Add onions and sauté for 2 to 3 minutes.: In the pan the butter will shimmer and give off a rich, dairy aroma, then the diced onion added will sizzle, releasing sweet, savory steam as it softens. Cook until the onion becomes translucent and just starting to color for maximum sweetness without bitterness; those gentle brown edges deepen flavor. Turn the heat down if the onions are browning too quickly, since burnt onion imparts a sharp edge that overwhelms the filling. You should hear a gentle, steady sizzle.
Add salt, pepper, thyme, and poultry seasoning. Sprinkle flour on top and cook for 1 minute, stirring to evenly cook the flour.: Once spices such as salt , ground pepper , dried thyme , and poultry seasoning mingle with the softened onion , sprinkle the all purpose flour evenly and stir quickly; it will absorb fat and begin to smell toasty. Cooking the flour for about a minute removes rawness and builds the roux base that thickens the sauce. Stir constantly to avoid lumps, which are a typical problem if flour is dumped in too fast. A short nutty scent indicates readiness.
Gradually whisk in chicken broth and then milk. Add potatoes and carrots and let simmer for a few minutes to thicken. Check for seasoning and add more salt and pepper if desired.: As you whisk in the chicken broth slowly, the mixture will loosen into a glossy sauce, and adding the milk after incorporates creaminess, smoothing the texture. The simmer encourages the starch from the potato to meld with the roux, thickening the sauce to a spoon coating consistency. Stir and watch for small bubbles around the edges and a steady steam; that indicates proper simmering. Taste and adjust salt and ground pepper since seasoning can shift as the sauce concentrates. If it becomes too thick, a splash of broth corrects it.
Stir in chicken and peas. Turn heat off.: When you fold in the shredded chicken and peas , the filling cools slightly, and you will hear small settling sounds as the ingredients integrate into the hot sauce. This step finishes the composition, preventing overcooking of the delicate peas and keeping the chicken tender. Turning the heat off preserves texture and stops the sauce from breaking. A common mistake is continuing to cook here which can make the peas mushy and the chicken dry.
Fit 1 pie crust into the bottom of a deep dish pie plate. Pour filling into pie shell.: Laying the first refrigerated pie crust into the Deep Dish Pie Plate should feel cool and slightly firm, making it easier to press into corners without tearing. When the filling is spooned in, notice how it settles and how the surface becomes glossy, indicating correct thickness. Work quickly so the crust stays chilled, because a warm crust can shrink away from the dish during baking. If the crust tears, press gently to mend it rather than stretching.
Place second pie crust on top and trim excess. Press the two pie crusts together to seal and crimp edge using your fingers.: Setting the top crust over the filling, you will feel it drape and conform, and as you trim and press edges, the crusts should bond without gaps. Crimping seals the steam inside so the filling cooks evenly and the top browns attractively. Avoid overhandling which warms the pastry and can make it tough; keep your fingers cool and work briskly. Ensure the seam is sealed to prevent leaking.
Brush egg wash on top of the pot pie and use a knife to cut 4 slits to let steam escape.: Brushing the egg wash composed of an egg and water creates a glossy, evenly browned surface that catches the eye. The four slits let steam escape in a controlled way, preventing the pie from rupturing and allowing the filling to finish cooking. The sound of brush strokes on the crust and the glossy reflection are small sensory satisfactions. Cutting too many slits or too large openings can dry out the filling.
Place on a baking sheet and place in oven and bake for 30 minutes.: Sliding the pie onto a baking sheet makes it easy to handle and catches any occasional spills, then as it bakes for about 30 minutes the crust will turn a deep golden brown, and you may hear a faint crackle as butter layers crisp. The aroma of baked pastry and roasted chicken will fill the kitchen, and steam will vent from the slits. A common mistake is removing the pie too early, which leaves the filling overly loose and the crust pale, so wait until the top is deeply colored and the center is bubbly.