1. Prepare the puff pastry layers

Preheat the oven to 375°F. Gently roll two puff pastry sheets so each fits an 8 × 11-inch baking dish. Prick the pastry all over with a fork.

Place one sheet on parchment, cover it with another sheet of parchment, and set a second baking sheet on top. Bake for 10 minutes. Remove the top baking sheet and upper parchment, then bake for another 8–10 minutes, or until golden, crisp, and cooked through.

Repeat with the second sheet. Let both baked pastry layers cool. Keep the third sheet refrigerated for the top of the casserole.

2. Cook the chicken and potato

Season the chicken with ½ teaspoon kosher salt and ¼ teaspoon black pepper.

Heat the oil in a large skillet over medium heat. Add the chicken and cook until browned and cooked through. Transfer to a plate.

Meanwhile, place the diced potato in a small saucepan and cover with water. Bring to a gentle simmer and cook for about 6–8 minutes, or until just barely tender but still firm enough to hold its shape. Drain well and set aside.

3. Make the creamy filling

Melt the butter in the same skillet over medium heat. Add the onion and cook for 3–4 minutes, or until softened.

Sprinkle in the flour and cook, stirring, for 1 minute.

Gradually whisk in the chicken broth. Add the heavy cream, thyme, and remaining ¼ teaspoon black pepper. Cook for 4–6 minutes, stirring often, until the gravy is thick enough to coat a spoon.

Fold in the cooked chicken, drained potato, and frozen peas and carrots. Cook for another 2–3 minutes, until heated through. Stir in the remaining ½ teaspoon salt, then taste and adjust the salt and black pepper as needed.

4. Assemble the pot pie lasagna

Spread one-third of the chicken filling evenly in an 8 × 11-inch baking dish.

Add one baked puff pastry sheet, followed by another third of the filling. Add the second baked pastry sheet, then spread the remaining filling over the top.

Place the pastry layers gently without pressing them down.

5. Add the top pastry

Place the remaining uncooked puff pastry sheet over the filling. Trim or tuck the edges as needed and cut a few small slits in the pastry to allow steam to escape.

Beat the egg with 1 tablespoon water and brush the egg wash over the top.

6. Bake

Bake at 375°F for 20–25 minutes, or until the top pastry is puffed, deeply golden, and cooked through.

Let the pot pie lasagna rest for 10 minutes before cutting and serving.