• If using ready-made crust, you will need 1 crust rolled into a 11 to 12-inch circle on a lightly floured surface. Cover and make the filling.
• If making your own pastry, follow the instructions below.

For the Pastry

• Combine the flour, sugar, and salt in a bowl (or the bowl of a food processor). Cut in the butter until it's the size of peas.
• Add ice water all at once, and stir to combine until the dough comes together. If you're using a food processor, add the water to the feed tube and pulse until it comes together. Form the dough into a disk shape, wrap, and chill for about 30 minutes.

For the Filling

• In a small bowl or lidded jar, whisk 3 tablespoons of water with the cornstarch; set aside.
• In a large saucepan, combine the rhubarb, strawberries, sugar, orange zest, juice, and fresh mint sprigs if using. Cook over medium heat until some juices begin to release, about 5 minutes. Stir in the cornstarch slurry and bring to a boil; reduce the heat and simmer for 2 minutes. Remove the mint sprigs and refrigerate for 30 minutes to cool.

Assembling the Tart

• Preheat the oven to 425 degrees F and line a sheet pan with parchment paper.
• On a lightly floured surface, roll the chilled pastry into an 11 to 12-inch circle and transfer it to the parchment paper.
• Spoon the filling into the middle of the pastry, leaving a 1 1/2-inch border. Fold the border up and over the filling, pleating when necessary and pressing together lightly.
• Brush the pastry with egg wash and a sprinkling of turbinado sugar.
• Bake for 30 to 35 minutes, or until the pastry is browned and the filling has thickened. Cool for 30 minutes before slicing and serving.