Make dough. Let it rest, tightly wrapped in plastic and refrigerated, for a few hours or overnight.

Mix the sugars, cardamom, and cornstarch in a small bowl. Set aside.

Add the vanilla extract, lemon juice, and pinch of salt to the peach slices. (I'm a bit OCD about measuring how wide my peach slices are. You don't have to be so exact. As long as they are thick enough to withstand gentle mixing and not so thick as to be chunky, you'll be fine.) Mix until combined.

Add the sugar mixture to the peach slices and gently combine until the slices are well coated. Set aside while you prepare the galette dough.

On a parchment sheet roll the dough into a circle with a diameter approximately 12-inches/30 cm. Move the parchment onto a large (1/2 sheet pan) cookie sheet and refrigerate for at least 30 minutes.

After the 30 minutes, take the dough out of the refrigerator. Drain any excess juice from the peach slices. Paint a 10-inch/25 cm circle on the dough with egg white. Place the peach slices in a pretty pattern, starting at the center and working to the outside of that 10-inch/25 cm circle, leaving a 2-inch/5 cm border for pleating.

Make pleats with the border, on top of the outside of the circle of peaches.

Paint the pleats with egg wash (the yolk, plus any leftover white, plus 1 -2 teaspoons of water), then dust the pleats with the sparkling and/or turnbinado sugar. Refrigerate or freeze the galette for 30 minutes to 1 hour.

Preheat oven to 400° F/200° C. Remove galette from refrigerator/freezer and bake for 35-45 minutes or until the juices still in the galette are bubbling and the crust is golden brown. (If the crust browns too easily, cover the pleats with aluminum foil, leaving open the center of the galette. Once the galette is done, remove it from the oven, dot the peaches with tiny pieces of butter and let it cool before slicing.