1. Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a large bowl, combine the flour, sugar, and salt. Add the chilled butter and use a pastry cutter or your fingertips to blend until the mixture resembles coarse crumbs.
3. Gradually add ice water, one tablespoon at a time, mixing just until the dough begins to come together. Avoid overworking it, as you want to keep the crust tender.
4. Form the dough into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
5. In another bowl, toss together the strawberries, rhubarb, sugar, cornstarch, vanilla extract, and lemon juice. Let the filling sit for 15 minutes to meld the flavors.
6. On a floured surface, roll out the dough into a 12-inch circle. Transfer it to the prepared baking sheet.
7. Spoon the fruit mixture onto the center of the dough, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as you go to create a rustic edge.
8. Brush the crust with the beaten egg and sprinkle with coarse sugar for extra crunch and sweetness.
9. Bake the galette for 35-40 minutes, or until the crust is golden brown and the filling is bubbly.
10. Allow it to cool slightly before slicing and serving. Enjoy the galette warm or at room temperature for the best flavor.

Remember, my abuela always said a pinch of love makes every dish special. Feel free to adjust sugar levels according to your taste, making it as sweet or as tangy as you like.