1. For the dough: Process flours, sugar, and salt in a food processor until combined, about 10 seconds. Scatter butter slices over flour mixture and pulse until broken into pea-sized pieces, 6 to 8 times. Transfer to a large bowl and break up any large chunks with your fingers, if necessary.
2. Sprinkle water over mixture and stir with a spatula until dough just comes together and no dry flour remains. Turn out onto counter and gently knead and shape into a 6"-wide disk. Wrap in plastic and refrigerate for at least 1 hour and up to 2 days.
3. Place a rack in the lower-middle of oven and heat to 400 degrees. On a lightly floured counter, roll dough into a rough 16" round, then trim to create a 15" circle. Carefully transfer to a parchment-lined baking sheet, gently folding edges over to fit if needed, and refrigerate while you prepare the filling.
4. For the filling: Halve plums, remove pits, and thinly slice crosswise into half-moon pieces. In a large bowl, combine plums, blackberries, 1/2 cup sugar, flour, and salt and toss gently until fruit is evenly coated.
5. Leaving a 3" border all around, gently pierce dough all over with a fork. Arrange fruit mixture over top of pierced area and sprinkle with diced butter. Fold border over edge of filling, overlapping every few inches to pleat, and press down gently to seal. Whisk egg and milk together to make the egg wash, brush over crust, and sprinkle with remaining 1 tablespoon sugar.
6. Transfer crostata to oven and reduce temperature to 375 degrees. Bake until filling is bubbling and crust is deep golden brown, 45 to 50 minutes, rotating halfway through. Let cool on baking sheet for 40 minutes. Serve warm or at room temperature.
Source: Hannaford fresh Magazine, July - August 2016