Step 1

Preheat oven to 375°F.

Step 2

In a 10-inch cast iron skillet, melt butter over medium heat. Add onion and celery; cook until soft (about 5 minutes). Add garlic and cook for 1 more minute, stirring constantly. Whisk in the flour; cook, whisking constantly, for 1 minute. Gradually whisk in the chicken broth and milk until smooth. Add the salt, pepper and thyme. Cook, whisking frequently, until thickened (about 4-5 minutes).

Step 3

Remove from the heat. Stir in the chicken, peas and carrots, and corn.

Step 4

On a lightly floured surface, roll puff pastry into a 10-inch square. Place the puff pastry over the hot chicken mixture, letting the ends extend over the sides of the skillet.

Step 5

In a small bowl, whisk together the egg and the water. Brush the egg wash on top of the crust. Sprinkle with salt and pepper. Brush the undersides of the puff pastry corners with the egg wash; press to the skillet.

Step 6

Cut 2-3 slits in the top of the crust to vent.

Step 7

Place the skillet on a rimmed baking sheet (to catch any filling that may drip over) and bake in the 375°F oven until the puff pastry is golden brown and the filling is bubbly, about 30-40 minutes. Let rest 10-15 minutes before serving.