In a bowl, combine the flour, oats, brown sugar, salt, and butter to make the crumble. The texture should resemble wet sand and it is ok if some butter chunks remain. You can also use a food processor to speed up this process. Press half of the mixture into the bottom a greased 9×9 baking dish. Set aside.
In a separate bowl, combine the sugar, cornstarch, cherries, lemon juice, and lemon zest to make the filling. Pour on top of the crust in the baking dish. Top the filling with the remainder of the crumble. Place in the fridge for 30 minutes.
Preheat the oven to 350F and bake for 45-50 minutes until the top is browned.