Preheat the oven to 375°F. Butter an 8 x 8 or 9 x 9 baking dish. Line the pan with a piece of parchment paper, cut so that it extends on two sides to easily remove it from the pan.
In a medium bowl, mix the flour, brown sugar, oats, and kosher salt. Cut in the butter with a pastry cutter or fork. As it incorporates, use fingers to knead until it becomes an even, consistent dough with no more dry bits of flour (this takes a while, so be patient!). Use your fingers to crumble it into small crumbles and measure out 1 1/2 cup for the topping.
Pour the remaining crumbles into the prepared pan, and press it into an even layer for the crust of the bars. Use the bottom of a measuring cup to press in the dough and make it flat and even.
In a medium bowl, mix the peeled and sliced apples, granulated sugar, cornstarch, lemon juice, and cinnamon and nutmeg. Spread them in an even layer over the crust.
Add the reserved crumbles over the top in an even layer.
Bake for 55 minutes to 1 hour, until the top and edges are golden brown. Remove from the oven and cool on cooling rack for 20 to 30 minutes, then lift out the bars with the parchment. If desired transfer to the refrigerator and for 1 hour, or simply cool to room temperature on the counter.
Once the bars are room temperature, cut into 16 even squares. Drizzle on Maple Glaze with a fork, if desired. Store bars up to 1 day at room temperature, 1 week refrigerated (with parchment paper between the layers if stacked; bring to room temperature before serving), or 3 months frozen.