Combine flour, sugar, and salt in a food processor and pulse for 5 seconds to blend. Add the cold butter cubes and pulse until the mixture resembles coarse crumbs with some pea-sized pieces of butter still visible—this creates a flaky crust. Add ice water one tablespoon at a time, pulsing briefly after each addition, until the dough just comes together when squeezed (you may not need all the water). Form the dough into a disk, wrap tightly in plastic wrap, and freeze for at least 1 hour. I find that really cold dough is key to achieving those gorgeous, crispy, flaky edges, so don't skip the freezing step.

While the dough freezes, combine the sliced strawberries, rhubarb chunks, golden brown sugar, granulated sugar, lemon juice, corn starch, vanilla extract, and ground ginger in a bowl. Toss gently until the fruit is evenly coated and the corn starch dissolves slightly into the juices. Let sit at room temperature so the flavors meld. The corn starch will thicken the filling during baking and prevent the galettes from becoming soggy, while the ginger adds a subtle warmth that complements the tartness of the rhubarb.

Remove the dough from the freezer and cut it into 4 equal pieces. Working with one piece at a time, place it between two sheets of parchment paper and roll out to an 8-inch circle, about 1/8-inch thick. Slide the parchment onto a baking sheet, then gently peel off the top layer of parchment. Spoon about 1/4 of the filling into the center of the dough circle, leaving a 1-inch border all around. Fold the dough edges up and over the filling, creating loose pleats and allowing some filling to peek through in the center—the rustic, imperfect look is part of the charm. Repeat with the remaining three dough portions. Once all galettes are shaped and on baking sheets, freeze them for at least 20 minutes (this prevents them from spreading too much in the oven).

About 20 minutes into the freezing time, preheat the oven to 375°F. In a small bowl, whisk together the egg yolk and water to create an egg wash. Remove the galettes from the freezer and brush the dough edges (not the filling) with the egg wash using a pastry brush, then generously sprinkle the coarse turbinado sugar over the dough edges. The egg wash creates a beautiful golden sheen and helps the sugar adhere, giving you those delicious caramelized, crunchy edges.

Bake the galettes at 375°F for 15 to 40 minutes, depending on your oven and how thawed the dough is. Start checking around 15 minutes—the galettes are done when the crust is deep golden brown and the filling is bubbling around the edges (you'll see a bit of the fruit juices peeking out). If the dough edges are browning too quickly, loosely tent the galettes with foil for the remaining bake time. Cool for 5 to 10 minutes on the baking sheet so the filling can set slightly before serving.