Prepare the crust: Preheat your oven to 350°F (175°C). Grease and line a 9×9-inch baking pan with parchment paper, allowing for some overhang for easy removal. In a large bowl, whisk together the flour, sugar, and salt. Add the cold butter cubes and use a pastry cutter or your hands to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in the egg yolk and cold water, mixing until the dough just comes together. Press the dough into an even layer in the bottom of the prepared pan.

1. Prepare the apple filling: Peel, core, and slice the apples into thin wedges. In a medium bowl, toss the apple slices with lemon juice, sugar, cinnamon, nutmeg, and flour. Stir until the apples are evenly coated.
2. Assemble the pie squares: Spread the apple filling evenly over the prepared crust.
3. Make the streusel topping: In a medium bowl, combine the flour, oats, brown sugar, cinnamon, and salt. Add the cold butter cubes and use a pastry cutter or your hands to work the butter into the dry ingredients until the mixture forms coarse crumbs. Sprinkle the streusel topping evenly over the apples.
4. Bake: Place the pan in the preheated oven and bake for 45-50 minutes, or until the topping is golden brown and the apples are tender. If the edges of the crust or streusel brown too quickly, cover the edges with foil to prevent burning.
5. Cool and slice: Let the Dutch Apple Pie Squares cool in the pan for about 15-20 minutes. Use the parchment paper overhang to lift them out of the pan, then transfer to a cutting board to slice into squares.
6. Serve and enjoy: Serve warm with a scoop of vanilla ice cream or enjoy at room temperature. These squares are perfect for any occasion!