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Stir rhubarb, raspberries and sugar together in a non-reactive saucepan and cook over medium heat, stirring occasionally, until juices begin to exude. Continue simmering, meanwhile, stir the cornstarch and cold water together in a small bowl.
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When fruit it nice and juicy and a bit tender but not yet soft, about 3 to 5 minutes of simmering total, stir in the cornstarch mixture. Boil for 1 minute, stirring well to combine, then remove from heat. Scrape into a bowl and refrigerate for about 30 minutes, stirring once or twice to release heat.
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Meanwhile, Position oven rack in upper third of oven. Preheat oven to 400°F/200°C.
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Roll out dough on a lightly floured piece of parchment paper to a large freeform round about 15-inches (38 cm) across. Scrape chilled filling into center of dough, leaving a broad 4-inch (10 cm) border. Using image to help guide you, flip the edges of the dough up and over the filling, leaving the center open. Slip parchment onto a baking sheet pan. Brush dough with a little beaten egg or lactose-free milk, if desired, and bake for about 40 to 50 minutes or until pastry is golden brown. Cool pan on rack for 10 minutes. Serve crostata warm or at room temperature. Crostata is best served the day it is made. A dollop of whipped cream or vanilla ice cream is a nice addition.