Prepare the Crust: Place 2 cups flour and salt in a food processor and pulse until blended. Add 1 cup butter and pulse until the butter is the size of peas. While pulsing, add just enough ice water (4-5 tablespoons) to form moist crumbs. Press this mixture onto the bottom of a greased 13x9-inch baking pan. Bake at 350°F until the edges are lightly browned, about 20-25 minutes, then cool on a wire rack.

Prepare the Apple Filling: In a large bowl, combine the sliced apples, 1 cup sugar, and cinnamon. Toss everything together until the apples are evenly coated with the cinnamon-sugar mixture. Spoon this apple filling evenly over the cooled crust.

Make the Streusel Topping: Place the remaining flour, butter, and sugar in the food processor and pulse until coarse crumbs form. Sprinkle this streusel mixture evenly over the apple layer, creating a crumbly topping that will become golden and crisp during baking.

Bake the Bars: Return the pan to the oven and bake at 350°F until the streusel topping is golden brown and the apples are tender, approximately 60-70 minutes. The apples should be soft when pierced with a fork, and the topping should be crisp and caramelized.

Cool and Serve: Allow the bars to cool completely on a wire rack before cutting. This cooling period is essential to allow the bars to set properly. Once completely cooled, cut into squares or rectangular bars for serving.