Adapted from a recipe by Karen DeMasco / Bon Appetit by way of Lottie and Doof.
To make the crust, combine both flours, sugar, and salt in a processor; blend then add butter and pulse until the butter is reduced to pea-size pieces. Whisk egg and milk in a small bowl; add to the processor and pulse until moist clumps form. Gather the dough into a ball; flatten into a disk. Wrap in plastic wrap and chill for an hour. (Can be made 2 days ahead.)
To make the filling, combine the rhubarb, raspberries, sugar and cornstarch in a large heavy saucepan with about 2 Tbsp water to help get it started. Cook over medium heat, stirring often, until sugar dissolves and juices are released, about 4 minutes. Increase the heat and bring to a boil (rhubarb will not be tender and slices will still be intact), then remove from the heat and cool to room temperature.
Preheat the oven to 400° F. Roll the dough out into a 12-inch round and place on a parchment-lined sheet; mound filling in the middle of the pastry, leaving 1 1/2-inch border around the edge. Gently fold the edges of dough over the filling, folding as it naturally folds. Brush the pastry with beaten egg and sprinkle with coarse sugar. Bake until crust is golden brown and filling is bubbly, about 45 minutes.
Let the crostata cool on baking sheet on a rack. Transfer to a cutting board, cut into wedges, and serve with whipped cream or ice cream. Serves 6-8.