In a food processor, combine all-purpose flour, fine sea salt, and granulated sugar. Pulse until well mixed, then add very cold unsalted butter in chunks, pulsing until the mixture resembles pea-sized crumbs. Gradually pour in ice water, pulsing until the dough begins to clump together. Form it into a disk, wrap in plastic, and refrigerate for at least 1 hour.

In a large mixing bowl, combine trimmed and sliced fresh rhubarb with hulled and quartered strawberries. Add granulated sugar, cornstarch, lemon zest, and freshly squeezed lemon juice, gently folding until the fruits are evenly coated. Let this mixture macerate for 15 minutes.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

On a lightly floured surface or on the parchment, roll out the dough to a 13-inch round, about 1/8 inch thick. Transfer the rolled dough to the prepared baking sheet.

Spoon the fruit mixture into the center of the dough, leaving about 2 inches of dough around the edges. Gently fold the edges of the dough over the filling in overlapping pleats. Brush the crust with an egg wash and sprinkle turbinado sugar on top.

Bake in the preheated oven for 40-45 minutes. If the edges brown too quickly, cover them lightly with foil.

Once baked, allow the galette to cool for at least 30 minutes before slicing. Serve warm or at room temperature.