Preheat the oven to 350 degrees. Grease a 9x9 baking pan (or line with parchment paper).
Make the crust: In the bowl of a stand mixer (or a mixing bowl with a hand mixer), cream together the butter, sugar, and salt until light and fluffy (a couple of minutes). Add in the flour and cinnamon and mix until everything is well combined (the mixture will be crumbly).
Sprinkle the mixture into the prepared baking pan and press it into the pan and a little bit up the sides. Bake in the preheated oven for 15 minutes. Remove and allow to cool slightly.
Make the filling: In a mixing bowl, mix together the sliced apples, sugar, flour, and cinnamon until the apples are well coated. Set aside.
Make the topping: In a bowl, mix together the oats, brown sugar, flour, cinnamon, and salt. Add the butter and cut in with your fingers or a fork until the mixture resembles a course meal.
Put everything together: pour the apples on top of the crust and spread around evenly. Sprinkle the topping over the apples and press down gently.
Bake for 40-45 minutes, until it's gently bubbling and the topping is golden. Allow to cool before cutting and serving.