Preheat the oven to 425°F.
(Skip to step 4 if you’re using pre-made pastry dough) In a food processor, combine butter, salt and flour. Pulse until the mixture forms a large crumb. Add in about a tablespoon of ice water, pulse a few times, and repeat until mixture just starts to come together. NOTE: You may not quite need the entire ¼ cup.
Flour your hands and a working surface. Separate the pastry in half and roll each half into a circle using a rolling pin (a floured jar/wine bottle with the label removed works just fine) until about a quarter-inch thick.
(Start here if using pre-made dough) Grease a six-inch round baking dish and place half of pastry dough inside, smoothing it out to ensure it covers the walls and lies as flat as possible. Place the second half of the pastry dough on a plate. Place the baking dish and plate in the refrigerator.
Add olive oil to a large pan under medium-high heat and cook onion until it begins to turn translucent. Add celery and carrots and let cook for five minutes before adding frozen vegetables. Cook until frozen vegetables have mostly thawed.
Turn heat up to high and add chicken stock, cream, flour, thyme, and salt and pepper, to taste. Let the mixture cook for about 10 minutes, stirring occasionally to ensure nothing sticks. Stir in chunks of turkey.
Remove the pastry dough from the refrigerator. Scoop the filling into the baking dish, and flatten mixture to ensure it is as even as possible. Top with second sheet of pastry dough, removing any excess and crimping the edges with your fingers.
Bake for 15 to 20 minutes, until golden brown. Let filling set for about 10 minutes and enjoy!