Part 1: Cooking the Rich and Creamy Filling

1. Melt and Sauté: In a large Dutch oven or heavy-bottomed pot, melt the 4 tablespoons of butter over medium heat. Once it’s shimmering, add the diced onion, carrots, and celery. Sauté the vegetables for about 8-10 minutes, stirring occasionally, until they have softened and the onion is translucent. This step builds the foundational flavor base, so don’t rush it!
2. Create the Roux: Sprinkle the 1/3 cup of all-purpose flour over the softened vegetables. Stir continuously for about 2 minutes. The flour will coat the vegetables and cook slightly, which is crucial for getting rid of any raw flour taste. This mixture of fat (butter) and flour is called a “roux,” and it’s the secret to a thick, luscious sauce.
3. Build the Sauce: Slowly, while whisking constantly, pour in the 2 cups of chicken broth. Pouring slowly and whisking prevents lumps from forming. Once the broth is incorporated, the mixture will begin to thicken. Now, switch to a spoon and bring the mixture to a gentle simmer.
4. Add the Creaminess: Reduce the heat to low and stir in the 1 cup of heavy cream or half-and-half. Let it gently warm through, but do not let it boil after adding the cream.
5. Combine the Final Ingredients: Remove the pot from the heat. Stir in the 3 cups of shredded cooked chicken, 1 cup of frozen peas (no need to thaw them), 1 teaspoon of dried thyme, salt, and freshly ground black pepper. If you’re using it, add the chopped fresh parsley. Give everything a good stir to combine.
6. Taste and Adjust: This is your last chance to perfect the filling. Taste it and adjust the seasoning if necessary. Does it need a pinch more salt? A bit more pepper? Now is the time to add it.
7. Transfer to Baking Dish: Pour the finished filling into a 9×13-inch baking dish or a similar 3-quart casserole dish. Spread it out into an even layer. Set aside while you prepare the topping.

Part 2: Crafting the Puff Pastry Ghosts

1. Preheat Your Oven: Position a rack in the center of your oven and preheat it to 400°F (200°C).
2. Prepare the Pastry: Gently unfold your thawed sheet of puff pastry on a lightly floured work surface. If it seems sticky, sprinkle a tiny bit of flour on top. Use a rolling pin to gently roll it out just a bit, to smooth any creases.
3. Cut Out Your Ghosts: Using a ghost-shaped cookie cutter (about 3-4 inches tall), cut out as many ghosts as you can from the puff pastry. You should be able to get 8-10 ghosts. Don’t worry about spacing them perfectly on the baking dish yet. You can re-roll the scraps once to cut out a few more smaller ghosts if you wish.
4. Arrange the Topping: Carefully arrange the puff pastry ghosts on top of the chicken pot pie filling in the baking dish. You can overlap them slightly. They will puff up as they bake, creating a beautiful, cohesive topping.
5. The Golden Touch: In a small bowl, whisk together the egg and 1 tablespoon of water to create an egg wash. Using a pastry brush, gently brush the tops of the puff pastry ghosts with the egg wash. This is what will give them that gorgeous, deep golden-brown color and a beautiful shine. Be careful not to let the egg wash drip down the sides of the pastry, as it can seal the layers and prevent them from puffing up to their full potential.
6. Give Them Eyes: Using your fingers or a pair of tweezers for precision, place two black sesame seeds or poppy seeds on each ghost to create the eyes.
7. Bake to Perfection: Place the baking dish on a baking sheet to catch any potential drips. Bake in the preheated 400°F oven for 20-25 minutes, or until the puff pastry is puffed, golden brown, and cooked through, and the filling is hot and bubbly around the edges.
8. Rest and Serve: Carefully remove the Ghost Pot Pie from the oven. Let it rest for at least 10 minutes before serving. This is a crucial step! It allows the filling to set up and thicken slightly, so it doesn’t run all over the plate.