Prepare the apple filling by melting unsalted butter in a medium saucepan over medium heat. Add chopped apples, light brown sugar, flour, cinnamon, water, and salt. Cook until apples soften and caramelize slightly, about 5–7 minutes.
In a large mixing bowl, combine all-purpose flour, rolled oats, brown sugar, baking powder, and salt. Mix well. Add cut pieces of unsalted butter and work into the dry ingredients until crumbly but chunky.
Preheat oven to 350°F (175°C). Grease a baking dish or line with parchment paper. Press half of the oatmeal mixture into the bottom of the dish. Pour warm apple filling over this layer and spread evenly. Crumble remaining oatmeal mixture on top.
Bake for approximately 30–35 minutes until golden brown on top. Allow to cool completely in the pan before cutting into squares.