2 1/2 cup cake flour, plus more for dusting
3 tbsp sugar
1/2 tsp salt
Grated zest of 1 lemon
1 cup (2 sticks) cold unsalted butter, cut into small chunks
1/4 cup ice water, plus more if needed
5 large ripe peaches, pitted and cut into 16 slices each
2 pint fresh blackberries
Juice of 1 lemon
3 tbsp sugar
1 tbsp all-purpose flour
2 tbsp honey
1 large egg, at room temperature
1 cup mascarpone or freshly whipped cream
Make the dough:
Combine the flour, sugar, salt, and lemon zest in a food processor. Add the butter and zap quickly until the mixture resembles coarse crumbs. Add the ice water, 1 tablespoon at a time, and zap until the dough just forms a ball without being too wet or sticky. Remove the dough from the processor and pat it into a flat disk, about 2 inches thick. Wrap it into plastic wrap and refrigerate for at least 30 minutes.
Make the filling:
Combine the peaches, blackberries, lemon juice, sugar, and flour in a large mixing bowl. Toss the mixture gently to coat the fruit.
Preheat the oven to 365F.
Remove the dough from the refrigerator and allow it to soften enough to roll out.
Sprinkle your work surface and a rolling pin lightly with cake flour. Roll the dough out into a 16-inch round, about 1/4 inch thick. If the dough tears, press the edges back together. Transfer the dough to a 12 1/2-inch fluted quiche or tart pan with a removable bottom, allowing the excess to hang over the edges. Spoon the fruit mixture into the dough. Fold in the overhanging edges of the dough, leaving a 6-inch area of fruit exposed in the center. Press the crust into the rim of the fluted pan to form an edge.
Place a heatproof bowl over a saucepan of simmering water (or use a double boiler), and heat the honey in the bowl until it is thin. Add the egg and whisk them together. Brush this mixture liberally over the fruit and crust. Place the tart pan on a baking sheet, and bake for 1 hour and 10 minutes or until the crust is golden brown and fruit is bubbly. Remove it from the oven and let it cool slightly.
Serve the crostata warm or at room temperature, with a dollop of mascarpone or whipped cream in each serving.