Make crust: In a bowl mix flour and salt. Using a pastry blender or two knives, cut in butter until mixture is crumbly. Add 1/4 cup ice water; stir just until dough comes together, adding more water if needed. Gather dough into a disc, wrap in plastic wrap and chill at least 30 minutes.
Make filling: Peel, core and thinly slice apples. Toss with blackberries, 1/2 cup sugar and cornstarch.
Place a rack in center of oven; preheat oven to 400°F. Remove dough from refrigerator and let soften. Roll out to a 14-inch circle; transfer to a parchment-lined baking sheet. Spread jam in a 10-inch circle on dough. Pile fruit over jam. Using your hands, carefully bring crust edges over fruit, folding where needed (pastry will not cover fruit). Brush crust with egg white; sprinkle with remaining 1 Tbsp. sugar.
Bake until fruit is bubbling and crust is golden, 35 to 40 minutes. Let cool for at least 10 minutes before serving.
There was too much filling to fit into the crust so I ended up making it a pie instead. The flavor was excellent.