Preheat Oven:

Preheat your oven to 350°F (175°C). Grease a 9×9-inch or similar-sized baking dish.

Prepare the Filling:

In a large mixing bowl, combine the sliced apples, granulated sugar, brown sugar, lemon juice, cinnamon, flour, and salt. Mix well to coat the apples evenly.

Pour the apple mixture into the prepared baking dish and spread it out evenly.

Make the Crumble Topping:

In a separate bowl, whisk together flour, oats (if using), brown sugar, cinnamon, and salt.

Add the cold butter cubes and use a pastry cutter, fork, or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.

Assemble:

Sprinkle the crumble topping evenly over the apples, covering them completely.

Bake:

Bake in the preheated oven for 40–45 minutes, or until the topping is golden brown and the apple filling is bubbling.

Serve:

Let cool slightly before serving. Best enjoyed warm with a scoop of vanilla ice cream or a dollop of whipped cream!