How to Make It
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.