How to Make It
MAKE THE PASTRY: In a food processor, pulse the flour, sugar and salt. Add the butter and pulse until it's the size of small peas. Add the water and pulse just until a dough forms. Turn out onto a work surface and gather the crumbs together. Shape into a disk, wrap in plastic and refrigerate for 1 hour.
MAKE THE FILLING AND STREUSEL: In a medium bowl, mix the flour with the walnuts, granulated and brown sugars, 1/4 teaspoon of the salt and the cinnamon. In a small skillet, cook 5 tablespoons of the butter over moderately high heat until golden brown, 2 minutes. Pour the butter over the walnut mixture and toss gently until crumbly. Refrigerate the streusel.
Preheat the oven to 375°. In a large skillet, bring the 1 cup of maple syrup to a simmer with the vanilla bean and seeds. Cook over moderately high heat for 3 minutes. Whisk in the remaining 2 tablespoons of butter and 1/4 teaspoon of salt. Add half of the apples to the skillet and cook until beginning to soften, 6 minutes. Using a slotted spoon, transfer the apples to a platter. Repeat with the remaining apples; let cool. Discard the vanilla bean and scrape the pan juices into a bowl.
On a lightly floured work surface, roll out the pastry to a 15-inch round, about 1/8 inch thick. Transfer to a baking sheet and refrigerate for 15 minutes. Brush the egg wash in a 2-inch band around the edge of the dough. Spread the apples on the pastry; add any pan juices to the bowl. Fold the border up over them, leaving the center exposed. Sprinkle the streusel topping over the apples in the center and brush the pastry with the egg wash. Bake the crostada for 50 minutes, or until golden brown. Let cool slightly, then serve with vanilla ice cream drizzled with the reserved pan juices and maple syrup.