Preheat your oven to 350°F (175°C). Cream together the softened unsalted butter and granulated sugar until light and fluffy. Gradually add the all-purpose flour and salt, mixing until a smooth dough forms. Press into the bottom of a greased 9x13-inch baking dish.

Peel and thinly slice the apples. Toss them with granulated sugar and cinnamon in a bowl. Once the crust is lightly golden, remove it from the oven and spread the apple mixture evenly over the hot crust.

In a separate bowl, combine rolled oats, all-purpose flour, and softened unsalted butter. Mix until crumbly and sprinkle over the apple filling.

Bake for 35-40 minutes until the crumble topping is golden brown and the apples are bubbling. Allow to cool in the pan for about 30 minutes.

Cut into squares and serve warm, optionally with a scoop of vanilla ice cream.