Preheat oven to 425°F. Press one pie crust into a 9-inch pie plate.
Melt butter in a saucepan over medium heat. Add onions and cook 2 minutes.
Whisk in flour, salt, pepper, and poultry seasoning. Cook until thick.
Slowly add broth and milk, whisking until thick and bubbly.
Stir in chicken and vegetables. Remove from heat.
Pour mixture into crust. Top with second crust, seal edges, and cut slits in top.
Bake 30–40 minutes until golden. Cover edges with foil in last 15–20 minutes.
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