Preheat your oven to 350°F. Grease an 8-inch square baking dish with butter or cooking spray.
Peel, core, and thinly slice the apples into slices around ¼ in thick, then place them in a large bowl.
Add brown sugar, lemon juice, cinnamon, and flour and stir to combine, then add to the prepared baking dish.
Topping
In a separate bowl, add the oats, flour, brown sugar, cinnamon, baking powder, and salt, then whisk to combine.
Add the cubed butter and cut it into the mixture with a pastry blender or two forks until small crumbles form.
Spread topping over apples and bake for around 45 minutes until the topping is browned and the filling is bubbly. Let cool for 15 minutes before serving.