In a bowl, whisk together flour, sugar, and salt. Add cold butter cubes and cut in using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.

Drizzle in ice water, one tablespoon at a time, mixing just until the dough holds together. Shape into a disk, wrap in plastic, and chill for at least 1 hour.

Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.

In a large bowl, toss strawberries and rhubarb with sugar, cornstarch, vanilla, and lemon zest. Let sit while rolling out the dough.

On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer to the prepared baking sheet.

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 needed.

Brush the crust with beaten egg and sprinkle with raw sugar, if using.

Bake for 40–45 minutes, or until the crust is golden and the filling is bubbling.

Cool slightly before serving. Delicious on its own or with a scoop of vanilla ice cream.