In a food processor add flour, cold butter, salt, vinegar, and cold water. Start with less water and add more if needed. Blend for a few seconds till it looks like coarse sand. To check if it holds together, take a bit in your hand and press it between your fingers, if it doesn't fall apart, it's ready.
Transfer the dough to a parchment paper. Work quickly so the butter doesn't melt. Form a disk about 1 inch thick and wrap the parchment paper around it. Transfer to the fridge for 30 munites or freezer till firm but still playable.
Meanwhile, cut the peaches into half-moon shape slices. Add the cornstarch, cinnamon, sugar, and vanilla. Mix to cat all the slices.
Take the chilled dough out and place it on a flat surface. Roll the dough into a rough circle about 1/4 inch thick. Don't worry about achieving a perfect shape; the rustic appearance of a galette is part of its charm.
Arrange the peach slices on top starting about 1½ to 2 inches from the edge.
Fold the edges. Brush with bitten egg and sprinkle sugar on top for an extra crunch. You can also sprinkle sugar over the peaches.
Transfer to a baking tray.
Bake on the middle rack at 400F (200C) for 35 minutes, or till golden brown. Take it out of the oven. Optional, you can brush the peaches with some honey for an extra shine. Wait to cool off a bit, slice, and enjoy. Add a scoop of vanilla ice cream on top, if you'd like. Enjoy!