Heat oven to 425°F
Melt the butter in a 12” inch cast iron (or other oven proof) skillet over medium heat
Add the onion, celery, carrots to the butter and sauté for 6 minutes, until the onions are tender and translucent*
Add the flour, salt, pepper, celery seed, garlic powder and thyme to the butter and vegetables creating a thick paste
Sauté for 2 minutes.Slowly add the milk
Once the milk is fully incorporated, slowly add the chicken stock
Simmer, whisking continuously until mixture has slightly thickened
Add the peas and chicken to the sauce
Roll out one pastry sheet into a 16-inch square on a lightly floured surface
Cut into 26 (3-inch) squares
Place the squares, slightly overlapping on top of the chicken mixture
In a small bowl, whisk the egg with 1 tbsp water
Brush the egg wash over the puff pastry
Place in the oven for 25 minutes until pastry is golden brown and filling is bubbling
If it is browning too quickly, cover with foil after it gets as dark as you'd like