Preheat oven to 350°F. Line a 9x9” pan with parchment paper to create a sling.
Make the crust: stir the melted butter and vanilla together. Add the sugar, salt and flour and mix until everything comes together. Press into prepared pan and bake for 15 minutes.
Meanwhile, prepare filling and streusel topping: Peel, core and cut apples into 1/4” thick slices. Whisk flour, sugar, cinnamon and nutmeg together. Sprinkle over apples and stir until apples are evenly coated.
For the Streusel topping whisk the oats, brown sugar, cinnamon and flour together. Cut in the cold butter until mixture resembles coarse crumbs.
When the crust comes out of the oven arrange apples over the hot crust, packing firmly. Sprinkle streusel topping over the apples then bake 30-35 minutes until golden brown.
Cool for 20 minutes at room temperature then refrigerate for at least two hours. Once chilled use the parchment sling to lift out of the pan. Cut into 16 squares or 20 rectangular bars.