Preheat oven to 400 degrees.
In a medium dutch oven or stock pot, melt butter and add cubed chicken, sauteeing until lightly cooked. Remove from pan and set aside.
Add chopped vegetables, chicken broth, parsley, salt, pepper, and bay leaf to the pan. Bring to a simmer and cook for 20 minutes.
Add corn starch and water to a small bowl or jar with a lid. Shake to combine and add to the pan. Add condensed soup mix, sour cream, and cheddar cheese, stirring to combine.
As the mixture cooks, prepare the dough. Roll out both piecrust disks until 1/4 inch thick or less, so much that it fits your pie plate.
Place one pie crust in your pie plate, forming the the dough in to the pan. Don't worry if the edges are uneven: you'll be crimping the crust from the top layer before it's baked. Press 1 teaspoon of celery seeds into the pie crust.
Pour mixture into the pie crust. Top with remaining pie crust, crimping the edges to seal. Cut 5 slits in the top. Brush egg on the crust and press remaining celery seeds into the top.
Bake for 30 minutes until golden brown. Allow to cool slightly before serving.