Preheat oven to 350 degrees F. Spray a 13 x 9" baking dish with cooking spray.
Peel the apples with a vegetable peeler.
Stand the apple up on a cutting board and cut each side around the core.
Place each apple piece cut side down and chop into 1" chunks.
In a large bowl, combine apples, granulated sugar, lemon juice, cornstarch, and 2 teaspoons ground cinnamon. Stir until combined.
Pour apple mixture into prepared baking dish and spread evenly.
Melt butter in a medium bowl in the microwave for 40-60 seconds.
Add oats, flour, brown sugar, remaining 1 teaspoon cinnamon, and salt to the melted butter and stir until combined.
Sprinkle crumble evenly over apple mixture.
Bake for 50-60 minutes until bubbly and golden brown. Cover loosely with foil if browning too quickly.
Cool on a wire rack for at least 15 minutes and serve warm. Enjoy!