Colorful peaches and berries make this tart look like a work of art.
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Adjust oven rack to lower-middle position and heat oven to 375 degrees. Line rimmed baking sheet with parchment paper and lightly spray with vegetable oil spray.
In large bowl, whisk cornstarch and ¼ cup sugar until well combined. Add peaches and blackberries. Use rubber spatula to gently stir until fruit is coated with sugar mixture.
Sprinkle a little flour over clean counter. Place dough on floured counter and sprinkle dough with a little extra flour. Use rolling pin to roll dough into 12-inch circle, rotating dough and reflouring counter in between rolls.
Use your hands to gently transfer dough to parchment-lined baking sheet. Use rubber spatula to scrape fruit mixture into center of dough. Spread into even layer, leaving 2-inch border around edges.
Fold 2-inch border of dough up and over edge of filling. Continue folding, overlapping folds of dough every 2 inches, until you get all the way around tart.
Place baking sheet in oven and bake until dough is golden brown and fruit is bubbling, 45 to 50 minutes.
Use oven mitts to remove baking sheet from oven (ask an adult for help). Place baking sheet on cooling rack. Let tart cool on baking sheet for 30 minutes.
Here’s how to pit and slice peaches: