Prepare the sourdough pie crust by combining flour, salt, sugar, and cold butter in a food processor until crumbly.
Add ½ cup cold sourdough discard, 1 egg yolk, and vinegar, then pulse until dough forms.
Shape dough into a disk, cover, and refrigerate while preparing filling.
In a saucepan, whisk together flour, chicken broth, milk, and sourdough discard over medium heat until thickened.
Add chopped potatoes, frozen veggies, cooked rotisserie chicken, thyme, onion powder, salt, and pepper; stir to combine and cook until potatoes are tender.
Preheat oven to 375°F (190°C).
Roll out the chilled dough on a floured surface to fit your pie dish.
Pour the chicken pot pie filling into the dish, then cover with the rolled-out dough.
Brush the crust with the egg white for a golden finish.
Bake for 45-50 minutes or until crust is golden brown and filling is bubbly.
Let cool for 10 minutes before serving.
