Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper. In a large bowl, combine the strawberries, rhubarb, 1/4 cup sugar, cornstarch, vanilla, and salt. Let stand for 15 minutes.
Unroll pie crust, roll it out slightly, and place it on the prepared baking sheet. Pour the fruit mixture onto crust, leaving a 2-inch border around the edges.
Fold the crust edges over the fruit. Overlap the dough where needed and press down gently. Brush the dough with the beaten egg. Sprinkle all over with the turbinado sugar. Bake until the crust is golden brown and juices are bubbly, about 50 minutes. Serve warm or at room temperature.