Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan by greasing it lightly with butter or lining it with parchment paper.

In a mixing bowl, cream together the unsalted butter and granulated sugar until fluffy, about 3 minutes. Add the all-purpose flour and salt, mixing until just combined. Press mixture into the prepared pan and bake for 15 minutes.

While the shortbread base is baking, toss the apple slices with lemon juice and cinnamon. Once baked, spread the apple mixture over the shortbread layer.

Mix together the rolled oats, brown sugar, and flour. Drizzle in melted butter, stirring until crumbly, then sprinkle over the apple layer.

Bake for 30-40 minutes until bubbly and golden brown. Let cool completely on a wire rack.

Once cooled, slice into squares and enjoy!