- 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.