1. Place the flour, sugar and salt in a food processor with a plastic dough blade. Pulse a few times to fully combine. Add the butter pieces and pulse until the mixture looks like little pebbles, 20-30 seconds. Pulse again while streaming the water through the feed tube. When the dough starts to form a solid ball, stop the processor. Transfer the dough to a clean, lightly floured surface and flatten evenly into a 2" thick disc (about 4-5” across). Wrap in plastic and chill in the refrigerator at least 1 hour.
2. Meanwhile, stir together the filling ingredients in a medium-sized mixing bowl. Let the mixture rest in the refrigerator until the dough is ready.
3. Preheat the oven to 425°F.
4. Dust a rolling pin and a clean surface with flour. Place the dough on the surface and roll out into a uniform circle to about 1/8" thick – the pastry should be about 11” across. (I use the side of my hand to even up and smooth the sides). Carefully roll the dough over the rolling pin and transfer to a rimmed baking sheet lined with parchment.
5. Stir the filling one more time, then spoon onto the center of the dough to about 1½" from the outer edge. Gently fold the edges of the dough up over the filling, pleating every 2-3” to enclose the crostata.
6. Whisk the egg in a small bowl. Brush evenly onto the top of the dough. Finish by sprinkling with the remaining 1½ teaspoons of Demerara sugar.
7. Bake for 50 minutes, until the crust is a beautiful golden brown and the juice from the fruit is thick and bubbly. Remove from the oven and let cool on the baking sheet at least 30 minutes.
8. Garnish with fresh rosemary branches.
9. When serving, cut into wedges. Serve with whipped cream and a sprig of rosemary. YUM!