Add cold butter, cut into small cubes, to the flour mixture. It's crucial that the butter is cold to achieve a flaky crust. Pulse the food processor until the mixture resembles coarse crumbs.

Now, incorporate your sourdough discard and vinegar. This is where the unique tangy flavor comes into play. Pulse the mixture until it just begins to form a dough. Be careful not to overmix, as this can lead to a tough dough.

Once your dough has come together, turn it out onto a surface and shape it into a disc. This shape makes it easier to roll out later. Wrap the disc in a piece of plastic wrap. Place the wrapped dough in the refrigerator. This step is vital as it allows the dough to firm up, making it easier to handle and helping to keep the butter cold, which is essential for a flaky crust. After chilling, take the dough out and place it on a lightly floured work surface. This prevents sticking. Using a rolling pin, roll the dough out to an even thickness. Aim for about ¼ inch thickness for a good balance between sturdiness and flakiness. Use a cookie cutter or the rim of a glass to cut circles out of the rolled dough. These will form the base of your hand pies."

In a large bowl, combine your filling ingredients, spinach (chopped if it's too bulky), crumbled feta cheese, and any optional herbs like dill or parsley. Mix them well.

Spoon a small amount of filling onto one half of each dough circle. Be careful not to overfill!

Fold the dough over the filling to create a semi-circle. Use a fork to crimp the edges. This not only seals the little pies but also gives a decorative edge.

Cut a small slit in the center to allow steam to escape.

Beat an egg and brush it over the top of each pie. This gives the pies a golden-brown finish.

Place the pies on a baking sheet lined with parchment paper.

Bake in a preheated oven at 374°F (about 190°C) for 20 to 25 minutes until golden brown.

Allow the pies to cool slightly on a wire rack. Serve the hand pies either as a main dish or as an appetizer!