01 - Preheat the oven to 350°F.
02 - In a large bowl, toss the sliced apples with sugar, cinnamon, lemon juice, and flour. Transfer the mixture to a greased 9-inch baking dish.
03 - In a separate bowl, combine flour, oats, brown sugar, and salt. Add cold butter and rub with your fingertips until the mixture resembles coarse breadcrumbs.
04 - Evenly sprinkle the crumble topping over the apple filling. Bake for 35 to 40 minutes, or until the topping is golden brown and the apples are bubbling.
05 - Allow the dish to cool slightly before serving. Serve warm, optionally with custard, cream, or vanilla ice cream.