Preheat oven to 400 degrees. In a saucepan over medium heat, melt butter and cook onions, carrots and celery until soft and translucent. Stir in flour, salt, pepper, celery seed and thyme. Slowly stir in chicken broth and milk. Simmer over medium-low heat until thick. Remove from heat and set aside.
Place the chicken mixture in an 8x8 glass baking dish. Cover with refrigerated pie crust. Seal edges and cut away excess dough. Make several small slits in the top to allow steam to escape.
Bake in the preheated oven for 30 minutes or until the pastry is golden brown and filling is bubbly. Cool for 10 minutes before serving.
Click here for recipes from the 2010 Foster Farms Fresh Chicken Cooking Contest!