Prepare an 8 by 8 inch pan with parchment paper and preheat an oven to 180c (350f).
Into a medium sized mixing bowl, add the flour, sugar, and salt, and mix with a spoon until combined.
Add the melted butter and vanilla extract and mix with a spoon until you get an homogonist mixture.
Transfer the dough to a parchment paper lined 8 by 8 inch pan and spread an even layer using your clean hands or a spoon.
Bake in a 180c (350f) preheated oven for 15-20 minutes or until the crust is lightly golden brown. Set aside to cool.
Peel, core, and cut the apples into thin slices. Into a large mixing bowl, add the apple slices, cinnamon, nutmeg, lemon juice, flour, and sugar. Mix with a spoon until well combined.
Make the crumble! Into a small mixing bowl, add the flour, oats, brown sugar, cinnamon, and mix with a spoon until well combined.
Add the melted butter and mix well.
Top the baked shortbread crust with the apple mixture and spread an even layer of it with a spoon.
Sprinkle the crumble on top. Bake in a 180c (350f) preheated oven for about 40 minutes or until the apples are cooked through and the crumble topping is golden brown and crispy.
Let it cool in the pan for at least an hour to prevent it from crumbling up. Drizzle with caramel sauce and serve!