Dissolve cornstarch in 3 tablespoons water in a small bowl; set aside.
2Combine 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.
3Stir 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.
4Preheat 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.
5Gently 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.
6Transfer crostata to a platter, cut into wedges, and serve with whipped cream or ice cream.