Prepare the Apple Filling. Peel, core, and slice the apples into thin wedges. Toss them with lemon juice, granulated sugar, cinnamon, and nutmeg. Arrange the apple mixture evenly in a baking dish.
Make the Crumble Topping. In a separate bowl, combine all-purpose flour, rolled oats, brown sugar, and a pinch of salt. Cut in cold, diced unsalted butter until the mixture resembles coarse crumbs.
Bake to Perfection. Sprinkle the crumble mixture evenly over the apples. Bake in a preheated oven until the topping is golden brown, and the apples are tender and bubbling. Allow it to cool slightly before serving.