In a large mixing bowl, combine flour, sugar, and salt. Cut in cold unsalted butter until resembling pea-sized crumbs. Gradually add ice water, mixing until the dough comes together. Form into a disk, wrap in plastic, and chill for at least 30 minutes.

In a separate bowl, toss diced rhubarb and hulled, quartered strawberries with lemon zest, sugar, and cornstarch. Set aside to allow flavors to meld.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Once chilled, roll the dough out into a 14-inch circle on a lightly floured surface and place it on the prepared baking sheet.

Center the fruit filling over the dough, leaving a 2-inch border. Fold the edges of the dough over the filling. Brush with melted butter and sprinkle turbinado sugar on top.

Bake for 35 to 45 minutes, until the crust is golden brown and the filling bubbles.

Allow the tart to cool on a wire rack for at least 15 minutes before serving.