Heat oven to 425°F. Lightly grease a 9-inch pie plate.
Heat oil in a large skillet over medium heat. Add vegetables and season with thyme, garlic powder, onion powder, salt and pepper. Cook until vegetables are thawed and starting to soften. Stir in chicken broth.
In a measuring jug, whisk together milk and cornstarch, then thoroughly stir into the vegetables as well. Simmer until gravy has thickened.
Take off the heat and stir in sour cream and butter. Fold in cooked turkey until smothered in gravy. Spread in prepared pie plate.
On lightly floured surface, unroll puff pastry. With rolling pin, roll out into 11-inch square. Cut off corners to make an 11-inch round. Place on top of pie filling, crimp edges, then cut 4 slits into the pastry. Brush with oil for more shine, if you like.
Bake pot pie in the hot oven for 20-25 minutes, until puff pastry is flaky and golden and filling is bubbly. Let sit on the counter for 10 minutes to allow the filling to thicken before scooping to serve.
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Notes
Ingredient notes
Cooked turkey: Use leftover turkey meat (light and dark), rotisserie chicken or just poach 1.5 pounds of turkey breast (or chicken breast) for this recipe. You can either shred or dice the cooked poultry - I prefer it diced in a pot pie.
Sour cream: You can use cream cheese as a substitute if you don't have sour cream on hand. I do not recommend using Greek Yogurt, as it tends to split easily.
Milk: Feel free to use heavy cream, evaporated milk or half-and-half in place of the milk.
Frozen vegetables: I like to use a classic blend of frozen mixed vegetables with corn, peas, carrots and green beans. California blend (with cauliflower) is also great.
Potatoes: Feel free to add a cup of diced potatoes to the filling to make it heartier.
Puff pastry: Pie crust or phyllo sheets are great alternatives for the puff pastry. You can also use canned refrigerated biscuits or even crescent roll dough. If you prefer a double crust on your pot pie, you can absolutely do so. I would recommend using a pie crust though, not puff pastry.
Recipe tips
Cornstarch slurry: Make sure to fully whisk the cornstarch into the milk, otherwise you'll end up with a lumpy gravy. For the same reason, stir the cornstarch slurry into the filling as soon as you add it; otherwise you'll have one big blob of seized up gravy in your filling and the rest won't thicken
Pastry: Keep the puff pastry chilled until you're ready to roll it out, it should be cold when it hits the oven - warm puff pastry is almost impossible to handle and doesn't turn out as flaky as it should. By the way, I bake puff pastry cut-offs as well and serve the kids while they wait for the pie to cool.
Filling: This makes a very creamy pot pie - that's how we like it best, as leftover turkey tends to be dry so the amount of gravy can help counteract this. If you want a less creamy filling, feel free to leave out the butter and sour cream.