If you're making the pie crusts from scratch, do that first and set the dough aside.

Bring a medium pot of water to a boil. Peel and dice the potatoes into 1/2" chunks. Add them to the boiling water and boil for 15 minutes. Drain and set aside.

In a large skillet, add the butter, onions, garlic, and celery. Over medium heat, sauté until the onions are translucent and fragrant.

Stir in the flour, salt, pepper, and celery seeds.

Slowly whisk in the broth. Whisk until the roux thickens, about 5 minutes.

Add in the frozen vegetables and stir to combine. Add the potatoes and stir again. If the mixture is too thick, add more broth, water, or milk to loosen slightly. You do want it to be thick enough to stay in place when spooned into the pie crust.

Divide pie crust into 3 portions and roll each portion into a circle, about 1/4" thick.

Cut each circle into 4 large pieces, like a pie. Spoon the filling diagonally into the center of each piece. See the photos above for reference.

Close the cut pieces of pie crust over the filling to form triangles and crimp the edges with a fork to seal.

Place on a parchment-lined baking tray and freeze for 15 minutes. This step is optional, but it helps the crust flake much better than if skipped.

Preheat the oven to 400°F. Add an egg to a small bowl and whisk until smooth. Add a dash of water and mix again to loosen.

Brush the tops with beaten egg if desired, right before baking. Bake fresh or frozen for 20 minutes, or until the crust is golden brown and fully baked.