Step 1: Prepare the Apples
Preheat your oven to 350°F (175°C) and lightly grease your baking dish.
In a large bowl, toss the apple slices with granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, and cornstarch until evenly coated.
Spread the apple mixture evenly in the prepared baking dish.
Step 2: Make the Topping
In a separate bowl, combine oats, flour, brown sugar, cinnamon, and salt.
Add the cold, cubed butter and use a pastry cutter or fork to blend it into the dry ingredients until the mixture resembles coarse crumbs.
Step 3: Assemble and Bake
Sprinkle the topping evenly over the apple mixture.
Bake in the preheated oven for 40–50 minutes, or until the topping is golden brown and the apples are bubbling.