A Simple Chicken Pie

I created this pie for dinner last night. Mr. Fox and I thought it was too good not to share! The ingredients I used are listed below -- feel free to experiment!

Ingredients for about 6 servings:
For the chicken:
Approximately 2 1/2 lbs cooked, shredded chicken -- I used a combination of chicken thighs and breasts.

For the vegetables:
A 16-ounce bag of mixed, frozen peas and carrots
2-3 cups diced potatoes (mine came frozen in a bag)

For the thick white sauce:
3 cups whole milk
1/2 teaspoon salt and grinds of black pepper
1 teaspoon onion powder
1/2 to 1 teaspoon dried tarragon leaves
4 generous tablespoons cornstarch blended with just enough water to make a smooth slurry

For the crust:
A sheet of frozen puff pastry dough, thawed but still quite cold (I used gluten-free puff pastry because Mr. Fox requested it)