How to Make It
Combine flour, 2 tablespoons granulated sugar, and 3/4 teaspoon salt in the bowl of a stand mixer fitted with the paddle attachment. Beat on low speed until combined. Increase speed to medium-low, and gradually add butter, beating until butter is in pea-size pieces, about 2 minutes. Drizzle 1/4 cup ice water into mixture, and beat on medium-low speed until dough comes together. Turn dough out onto a work surface, and shape into a disk. Wrap in plastic wrap, and chill for at least 1 hour.
Whisk together remaining 1 cup granulated sugar, 1/4 teaspoon salt, and cornstarch in a medium saucepan. Add 2 cups rhubarb. Cook, stirring constantly, over medium until sugar has melted and rhubarb begins to break down, about 8 minutes. Continue to cook, stirring often, until mixture has thickened, about 4 minutes. Place remaining 4 cups rhubarb in a large bowl. Pour cooked rhubarb over raw rhubarb. Add ginger and orange zest; stir until well combined.
Unwrap dough, and roll into a 12 1/2-inch circle (about 1/4 inch thick) on a lightly floured surface. Transfer dough to a parchment paper–lined baking sheet, and trim edges to form a 12-inch circle; discard scraps.
Spoon rhubarb mixture onto dough, leaving a 2 1/2-inch border. Use the parchment to help fold the dough edge up and over the filling, pleating as needed and pressing lightly to secure. Brush dough with egg wash, and sprinkle with turbinado sugar. Chill, uncovered, until dough is firm, about 1 hour.
Preheat oven to 425°F. Bake crostata until crust is golden and filling is thickened, 30 to 35 minutes. Cool completely, about 3 hours. Serve with Whipped Mascarpone.