1. Preheat oven to 375°. In a 13x9-inch baking pan, combine blackberries, blueberries, sugar, flour, salt, lemon juice, and cinnamon, tossing gently. Dot with butter.

2. On a floured surface, roll 1 portion of Buttery Pastry Dough into a 9x6-inch rectangle. Cut crosswise into 6 strips. Place strips crosswise over fruit mixture in pan, trimming edges if necessary. Roll remaining portion of dough into a 13x4-inch rectangle. Cut lengthwise into 4 strips. Place strips lengthwise in pan, trimming edges if necessary.

3. In a small bowl, whisk together egg and 1 teaspoon water; lightly brush over dough. Sprinkle with sugar.

4. Bake until crust is golden brown and filling is bubbly, approximately 45 minutes. Garnish with 1 teaspoon sugar, if desired. Let stand for 20 minutes before serving. Serve with vanilla ice cream, if desired.