In a saucepan, sauté onion in butter until soft and translucent.
Stir in flour, salt, and pepper until well blended with the butter and onions.
Gradually add chicken broth and milk, whisking constantly to prevent lumps.
Bring mixture to a boil, then cook and stir for 2 minutes until thickened.
Add turkey, cheddar cheese, and thawed peas and carrots. Cook until cheese melts completely. Set aside and keep warm.
For the crust, combine flour, celery seeds, and salt in a large bowl.
Cut in shortening using a pastry cutter until mixture resembles coarse crumbs.
Add cold water gradually until dough forms a ball.
Divide dough in half and roll out bottom crust to line a 9-inch pie plate, trimming even with edges.
Pour hot turkey filling into prepared crust.
Roll out remaining pastry for top crust and place over filling.
Trim, seal, and flute edges. Cut several slits in top crust for steam vents.
Brush top with milk for golden color if desired.
Bake at 375°F for 40-45 minutes until crust is golden brown.