In a mixing bowl, combine flour, sugar, and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Add ice water, one tablespoon at a time, mixing until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

In another bowl, combine strawberries, rhubarb, sugar, cornstarch, vanilla extract, and lemon juice. Toss to coat and set aside.

Preheat the oven to 375°F (190°C). Roll out the chilled dough on a floured surface into a 12-inch circle.

Transfer the dough to a baking sheet lined with parchment paper. Spoon the filling into the center, leaving a 2-inch border.

Fold the edges of the dough over the filling, pleating as necessary. Brush the crust with beaten egg and sprinkle with sugar.

Bake in the preheated oven for 40-45 minutes, or until the crust is golden and the filling is bubbly.

Allow to cool slightly before slicing and serving.