Preheat oven to 300 degrees F.
Generously spray a baking sheet pan (13"x18") with cooking spray and set aside.
In a mixing bowl, combine softened butter, flour, brown sugar and salt. Mix well until all ingredients are combined. (The mixture will appear dry.)
Scoop the crust ingredients into the prepared sheet pan and press firmly into the baking sheet to create an even, packed base.
Bake the shortbread crust for 15 minutes. Remove from heat and set aside.
While the crust is baking, peel the apples and cut them into small, bite-sized pieces. Place them in a large mixing bowl.
Drizzle the lemon juice over the apples and toss to coat evenly.
In a large saucepan, melt the butter over medium heat.
Once melted, add the apples, sugar, cinnamon, and nutmeg. Stir to combine.
Reduce the heat to medium-low and simmer for 7–10 minutes, or until the apples are tender.
In a small bowl, whisk together the water and cornstarch to make a slurry. Add the slurry to the apple mixture and stir until the filling thickens.
Pour the apple filling over top of the shortbread crust and spread evenly.
Transfer to oven and increase the temperature to 375 degrees F.
Bake for 15-20 minutes. The filling should be set around the edges and slightly jiggly in the center.
Remove from heat and let cool completely.
Drizzle with caramel if desired.
Cut into desired sized bars and serve!
Enjoy!