Step 1: In a large bowl, whisk together the flour, 1 tablespoon of sugar, and salt. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Step 2: Gradually add ice water, one tablespoon at a time, mixing until the dough just comes together. Form into a disk, wrap in plastic, and refrigerate for at least 1 hour.
Step 3: Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Step 4: In a medium bowl, toss the sliced rhubarb and strawberries with 1/2 cup sugar, cornstarch, and vanilla extract until evenly coated.
Step 5: On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer the dough to the prepared baking sheet.
Step 6: Pile the fruit mixture into the center of the dough, leaving a 2-inch border around the edges.
Step 7: Gently fold the edges of the dough over the fruit, pleating as you go to create a rustic border.
Step 8: Brush the edges of the crust with the beaten egg and sprinkle with turbinado sugar.
Step 9: Bake for 35-40 minutes, or until the crust is golden brown and the fruit filling is bubbling.
Step 10: Let the galette cool for at least 15 minutes before slicing to allow the juices to set.