Combine all-purpose flour, granulated sugar, and salt in a mixing bowl. Cut in cold, unsalted butter until the mixture resembles coarse crumbs. Gradually add cold water, stirring until the dough comes together. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

Peel, core, and slice apples. In a skillet over medium heat, melt unsalted butter, then add apple slices. Stir in brown sugar, ground cinnamon, cornstarch, and salt. Cook until mixture thickens, about 5-7 minutes. Remove from heat and let cool slightly.

Roll out two-thirds of the chilled dough, place it in a pie dish, and sprinkle almond flour on the bottom. Pour in the apple filling evenly. Roll out remaining dough for the top crust, cut into strips, and create a lattice pattern before crimping the edges.

Preheat oven to 425°F (220°C). Brush the lattice top with a mixture of beaten egg and water, then sprinkle with brown sugar. Cover edges with foil to prevent over-browning. Bake for 30 minutes, then remove foil and bake for an additional 30 minutes or until golden brown.

Remove pie from the oven and cool on a wire rack for at least 20 minutes. Serve warm with a scoop of vanilla ice cream on top.