01 - Set the oven to 350°F (180°C) to prepare for baking.
02 - In a large bowl, toss the sliced apples with lemon juice, granulated sugar, cinnamon, nutmeg, and flour until fully coated. Transfer to a greased 9-inch (23 cm) baking dish.
03 - In a separate bowl, blend flour, cold diced butter, brown sugar, oats, and salt using your fingertips or a pastry blender until mixture resembles coarse crumbs.
04 - Evenly distribute the crumble topping over the apple filling in the baking dish.
05 - Bake in the preheated oven for 35 to 40 minutes, or until the crumble topping is golden brown and the filling is bubbling.
06 - Allow to cool slightly before serving. Serve warm, optionally accompanied by vanilla ice cream or custard.