To make dough: In large bowl, whisk flour, sugar and salt. Add butter and toss with flour mixture. Smash each butter cube flat then use fork to stir in cold water. Knead lightly until dough comes together in ball. Transfer to floured work surface and use well-floured rolling pin to roll into 10-by-15-inch sheet. Fold each short side into middle and close like a book. Fold once more to make thick block and cut in half. Wrap each piece in plastic and chill at least 30 minutes or overnight.

To make filling: In large bowl, combine apples, grapes, sugar, flour, lemon juice and salt; set aside.

To assemble pie: Heat oven to 375 F.

Roll one half of dough out to 18-by-13-inch rectangle. Transfer to 10-by-15-inch heavy baking sheet; place in refrigerator to chill while rolling out top sheet. Roll out remaining dough to 18-by-13-inch rectangle and use pastry cutter to cut into 1 1/2-inch wide strips.

Transfer filling into chilled pastry and spread evenly. Lay dough strips on top of filling in lattice pattern and crimp sides together. Mix egg with water and brush top of pie. Sprinkle with demerara sugar and bake until filling is bubbling, and crust is nicely browned, about 45-50 minutes.