- 1 cup unbleached all-purpose flour
- 1/2 cup whole wheat flour
- 1 1/2 tablespoons sugar
- 1/2 teaspoon kosher salt
- 3/4 cup (1 1/2 sticks) chilled unsalted butter, cubed
- 1 large egg
- 1 tablespoon whole milk
- 1/4 cup cornstarch
- 4 cups 1/2″-thick slices rhubarb
- 1 6 ounce container fresh raspberries
- 1/3 cup sugar
- 1 large egg, beaten
- Raw sugar
- Sweetened whipped cream or vanilla ice cream (for serving)
- Combine both flours, sugar, and salt in a processor; blend for 5 seconds. Add butter; pulse until butter is reduced to pea-size pieces. Whisk egg and milk in a small bowl to blend; add to processor and pulse until moist clumps form. Gather dough into a ball; flatten into a disk. Wrap in plastic wrap; chill at least 1 1/2 hours. DO AHEAD: Can be made 2 days ahead. Keep chilled.
- Dissolve cornstarch in 3 tablespoons water in a small bowl; set aside. Combine rhubarb, raspberries, and sugar in a large heavy saucepan. Cook over medium heat, stirring often, until sugar dissolves and juices are released, about 4 minutes. Stir in cornstarch mixture and bring to a boil (rhubarb will not be tender and slices will still be intact). Transfer to a bowl. Chill until cool, about 30 minutes.
- Preheat oven to 400°F Roll out dough on floured parchment paper to 12″ round; brush with beaten egg. Mound filling in center of crust; gently spread out, leaving 1 1/2″ border. Gently fold edges of dough over filling, pleating as needed. Brush border with egg; sprinkle with raw sugar. Slide parchment with crostata onto a large rimmed baking sheet and bake until crust is golden brown and filling is bubbly, about 45 minutes. Let crostata cool on baking sheet on a rack. Transfer crostata to a platter, cut into wedges, and serve with whipped cream or ice cream.