• Preheat oven to 400° Fahrenheit. Spray 9x9 baking dish with neutral-flavored spray oil, coating bottom and all sides evenly. Set baking dish aside.

For the Chicken Pot Pie Filling

• Place large skillet on stovetop over medium heat. When skillet is warm, add 4 tablespoons unsalted butter and let butter melt completely, swirling and tilting skillet as needed to distribute oil across entire pan.
• When butter has melted, add ½ cup diced celery, ½ cup finely chopped carrots, ½ cup diced yellow onion, ½ cup frozen peas, and 2 small red potatoes to skillet. Stir to incorporate.
• Sauté vegetables until just tender but still crisp, approximately 5 minutes. Stir often.
• When vegetables are tender, sprinkle 4 tablespoons all-purpose flour over vegetables, then add 1 teaspoon garlic powder, 1 tablespoon dried thyme, 1 teaspoon dried whole sage leaves, 1 pinch nutmeg, salt, and freshly cracked black pepper. Stir to coat vegetables completely in spices and flour.
• Cook vegetables 1 minute, stirring constantly. After 1 minute, remove skillet from heat and let cool slightly.
• When pan has cooled, slowly add 1 cup whole milk (at room temperature) and 2 cups low-sodium chicken broth to skillet in small increments, stirring well after each addition. Once all milk and chicken broth has been added, return skillet to heat.
• Add 2 cups chopped, roasted chicken and stir to incorporate. Taste mixture and add salt and/or pepper if desired. When satisfied with flavor, transfer mixture to prepared baking dish and set aside.

For the Pastry Crust

• Lightly flour flat work surface, then carefully unfold 1 sheet ready-to-bake puff pastry dough and lay dough on floured surface.
• Gently press together seams of pastry dough. If needed, lightly flour rolling pin, then roll pastry dough out until just large enough to cover top of filling completely.
• Carefully lift pastry dough from work surface and place on top of pot pie filling.
• Crack 1 large egg into small mixing bowl, then add 1 teaspoon water. Whisk egg and water together until fully incorporated and egg is no longer streaky.
• Brush egg wash evenly over top of pastry dough.
• Use sharp knife to carefully cut 4 to 6 vent slits into top of pastry dough.

To Bake the Chicken Pot Pie Casserole

• After venting pastry dough, place baking dish in preheated oven. Bake casserole 30 minutes, or until puff pastry is golden in color and filling begins to bubble.
• Once pastry is fully baked, carefully remove baking dish from oven and set aside. Let casserole rest 5 to 10 minutes.
• After 5 to 10 minutes, portion casserole into desired servings. Serve warm.