Preheat your oven to 350°F and line a 13x9-inch baking pan with parchment paper for easy removal later.
Cream the softened butter and sugar in a large mixing bowl with electric hand mixers or in the bowl of a stand mixer until light and airy. About 3-4 minutes.
Whisk together the flour and cinnamon in a separate bowl.
Blend the dry ingredients gradually into the butter mixture until well combined.
Press the dough into the base of the prepared pan, spreading it evenly.
Bake the crust for about 15 minutes, or until the edges begin to turn golden.
Allow the crust to cool slightly while you prepare the filling and topping.
Toss the apple slices with the sugar, flour, and cinnamon in a bowl until well coated.
Mix the oats, brown sugar, flour, and cinnamon in a separate bowl.
Cut in the cold butter using a pastry cutter or fork until the mixture becomes crumbly.
Spread the apple filling evenly over the partially baked crust.
Sprinkle the crumble topping over the apples, pressing it down lightly so it sticks.
Bake for 55-60 minutes or until hot and bubbly and golden brown on top. The apples will be soft and jammy, and bubbling around the outside.
Cool completely in the baking dish and lift out with the help of the parchment paper and cut into squares or bars.
