Preheat oven to 350 degrees F. Grease a 9x13- inch baking pan with nonstick cooking spray. Set aside.
First, make the apple crisp topping. In a medium bowl, combine flour, oats, brown sugar, granulated sugar, cinnamon, and salt. Mix in the cold butter pieces with your fingers or a pastry cutter until it comes together and large clumps form. Set the topping aside. If it is warm in your house, you can put the topping in the refrigerator while you prepare the filling.
In a large bowl, combine the apple slices, lemon juice, sugar, cinnamon, vanilla extract, and corn starch. Stir until the apples are well coated.
Pour the apple mixture evenly into the prepared pan. Sprinkle the topping evenly over the apples. Bake for 45 minutes or until the topping is golden brown and the apples are soft and bubbling around the edges.
Let the crisp cool for 10 minutes. Serve warm with vanilla ice cream, if desired.
We like to use Granny Smith apples for this recipe because they are crisp and tart. You can use a mix of apples, Honey Crisp and Fuji apples are good options. The lemon juice will keep the apples from browning.