Place apple slices in a large bowl and gently mix in sugar, cornstarch, and cinnamon. Set aside.
3. Step
Preheat oven to 350˚F (180˚C).
4. Step
In another bowl, combine flour, oats, brown sugar, baking soda, salt, and butter. Mix until crumbly.
5. Step
Line a 9x13-inch (23x33 cm) baking dish with parchment paper and pour ⅔ of the oats mixture in the bottom of the pan, pressing firmly to pack it down to make a crust.
6. Step
Pour apples over the crust and spread in an even layer. Pour the remaining oat mixture over the top and spread evenly.
