For crust For filling
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. The crust can be made 2 days ahead. Keep chilled.
Make fillingDissolve 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.
Makes 8 to 10 servings.