Preheat oven to 400℉.
Remove the puff pastry from the freezer and let it thaw on the counter a bit while you prepare the filling.
1 sheet puff pastry
In a skillet, melt the butter over medium heat.
1/3 cup butter
Add the onion and sauté for 2-3 minutes.
1/2 cup diced onion
Sprinkle the flour on top and stir. Let the mixture cook for 30-60 seconds.
1/2 cup flour
Stirring constantly, slowly pour in the chicken broth. It'll thicken right away and thin out as you add more chicken broth.
1 1/2 cups chicken broth
Add the white wine (or additional broth) and milk and bring the mixture to a boil.
1/4 cup dry white wine, 1/2 cup milk
Remove from the heat and stir in the garlic, pepper, salt, vegetables, and chicken.
1/4 teaspoon garlic powder, 1/4 teaspoon black pepper, 1/2 teaspoon salt, 2 cups frozen peas and carrots, 2 cups diced or shredded chicken
Pour the filling into the pie plate.
In a small bowl, whisk together the egg yolk and water to create an egg wash; set aside.
1 egg yolk, 1 tablespoon water
Unroll the sheet of puff pastry and lay it over top of the filling; trim the excess puff pastry and crimp the edges, if desired.
Use a sharp knife to cut some vent holes into the puff pastry.
Brush the egg wash over the puff pastry.
Transfer the pie to the hot oven. I recommend putting a rimmed sheet pan below the pie plate in case it bubbles over, just to keep your oven clean!
Bake for 25-30 minutes, or until the puff pastry and golden brown and the filling is bubbly. (I like to check on the pot pie after about 20 minutes; if it's nice and golden brown on top, gently place a piece of foil over the pie so it doesn't get too dark on top.)
Let set for 5 minutes before serving.