Preheat oven to 375 degrees F. Line a 9x13-inch baking pan with foil or parchment paper, allowing excess to hang over pan edges. If using foil, grease or spray with non-stick cooking spray.
Make the Crust: Process the crackers, brown sugar and salt in a food processor until they have been reduced to fine crumbs, about 15 seconds. Add the melted butter and pulse to incorporate. Press the crumbs into an even layer in the prepared pan and bake until deep golden brown, 10 to 13 minutes. Cook on a wire rack for 20 minutes. Keep the oven on.
Make the Streusel: Whisk together the flour, sugars and cornmeal in a medium bowl. Add the melted butter and toss with a fork until the dry ingredients are all evenly moistened. Set aside.
Make the Filling: Combine the apples, melted butter, sugar, cinnamon and salt in a large skillet over medium-high heat. Cook, stirring occasionally, until the apples are soft and the liquid has evaporated, 8 to 10 minutes.
Assemble: Spread the apples over the crust in an even layer and sprinkle with the streusel. Bake until the streusel is brown, 25 to 30 minutes. Cool completely on a wire rack, about 2 hours. Using the foil or parchment overhangs, lift the bars from the pan and cut into squares. Store in an airtight container in the refrigerator or at room temperature.