Prepare the crust dough by whisking together flour, sugar, and salt in a large bowl. Add the cold butter cubes and use a pastry cutter or your fingers to work it into the flour until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Drizzle ice water over the mixture, one tablespoon at a time, gently stirring with a fork until the dough just begins to come together. Be careful not to overwork or add too much water.
Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes while you prepare the filling.
Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Make the filling by gently combining strawberries, rhubarb, sugar, cornstarch, lemon juice, vanilla, cinnamon, and salt in a large bowl. Toss carefully to avoid crushing the fruit.
Roll out the chilled dough on a floured surface into a 12-inch circle about ⅛-inch thick. Don’t worry about perfect edges – the rustic look is part of its charm!
Transfer the dough to your prepared baking sheet by carefully rolling it around your rolling pin and then unrolling onto the parchment.
Arrange the fruit filling in the center of the dough, leaving a 2-inch border all around. Avoid adding any excess liquid from the bowl.
Fold the edges of the dough up and over the filling, pleating as you go, leaving the center open to showcase the colorful fruit.
Brush the crust with beaten egg and sprinkle with coarse sugar for a beautiful, caramelized finish.
Bake for 40-45 minutes until the crust is golden brown and the filling is bubbling. If the crust browns too quickly, tent loosely with foil.
Allow to cool for at least 15 minutes before slicing to let the filling set up slightly. Serve warm or at room temperature, perhaps with a scoop of vanilla ice cream.