To make the crust, preheat the oven to 375 degrees. Lightly spray a 9-by-13-inch baking pan with cooking spray.
In a large bowl, using an electric mixer set on high speed, beat together the butter and granulated sugar until pale and fluffy, about 2 minutes. Reduce the speed to low and beat in about 1/2 cup of the flour and the salt, then add the remaining flour gradually and beat until the dough comes together in a soft ball. Transfer the dough to the prepared pan and, using your hands and a gentle touch, spread it out evenly over the bottom and slightly up the slides. Bake until the top is golden brown, 15-20 minutes. Transfer to a wire rack and let cool completely. leave the oven on.
to make the filling, place the butter and brown sugar in a large frying pan. Place over low heat and, using a wooden spoon, stir to melt the sugar into the butter to form a syrup. Add the apples, toss to coat with the syrup, and raise the heat to medium-high. Cook, stirring often, until the apples have softened and are beginning to caramelize in the syrup, 1-20 minutes. If the pan begins to burn, sprinkle in a little water, scrape the pan bottom, and reduce the heat. when the apples are very soft, stir in the cinnamon and nutmeg. Remove from eat and let cool completely.
Spread the apple filling over the baked crust in an even layer. Sprinkle the topping over and press lightly. Bake until the topping is golden, about 20 minutes. Let cool completely on a wire rack, then cut into bars and serve.