Boil peas, carrots, green beans, and celery in a saucepan until tender. Drain, set veggies aside.
Melt butter in a saucepan on medium heat and cook onion until translucent. Stir in 2/3 cup of flour as well as salt, pepper, celery seed and onion powder. Slowly whisk in chicken broth and milk until the mixture is smooth, comes to a simmer, and thickens. Remove from stove and mix in sweet potatoes.
Stir vegetables and turkey meat into the mixture. Drape pie crust in bottom of pie dish. Spoon filling into the crust, then top with the second crust.
Pinch the edges, poke some steam holes on top, and bake in the preheated oven until crusts are golden brown and the internal temperature of the pie is at least 165 degrees F. This should take about 30-35 minutes. If the crust starts to brown before the filling gets hot, cover the top crust gently with a sheet of foil. Cool before serving.