In a food processor, pulse together the all-purpose flour, fine sea salt, and granulated sugar until combined. Add very cold unsalted butter, pulsing until the mixture resembles pea-sized crumbs. Slowly drizzle in ice water, pulsing until the dough starts to clump together. Once formed, wrap it in plastic wrap and chill in the refrigerator for at least an hour to ensure a flaky crust.

While the dough chills, combine fresh rhubarb and hulled strawberries in a large mixing bowl. Sprinkle in granulated sugar, cornstarch, and lemon zest and juice, then gently stir to mix. Allow the fruit mixture to sit for 15 minutes.

Preheat your oven to 400°F (200°C) and prepare a baking sheet by lining it with parchment paper.

After the dough has chilled, place it on a lightly floured piece of parchment paper. Using a rolling pin, roll the dough into a rough 13-inch circle, about 1/8 inch thick.

Transfer the rolled dough onto the prepared baking sheet. Spoon the fruit filling into the center, leaving a 2-inch border around the edges. Fold the crust over the filling, creating pleats as you go.

In a small bowl, beat one egg until smooth and brush it generously over the exposed crust. Sprinkle turbinado sugar over the crust.

Place the assembled galette on the lower rack of the preheated oven and bake for 40 to 45 minutes until the crust is golden brown and the filling is bubbling.

Once baked, remove the galette from the oven and let it rest on a wire rack for 30 minutes.