Preheat the oven:
Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish or line it with parchment paper.
Prepare the apple filling:
Peel, core, and slice the apples into thin wedges. In a large bowl, toss the sliced apples with lemon juice, sugar, cinnamon, nutmeg, and flour until evenly coated. Transfer the mixture into the prepared baking dish.
Make the crumble topping:
In a separate bowl, combine the flour, oats, brown sugar, cinnamon, and salt. Add the cold, cubed butter. Using a pastry cutter, fork, or your fingers, cut the butter into the dry ingredients until the mixture resembles coarse crumbs. The pieces of butter should be pea-sized.
Assemble the crumble:
Evenly sprinkle the crumble topping over the apple filling, covering it as completely as possible.
Bake:
Bake the apple crumble in the preheated oven for 40-45 minutes, or until the topping is golden brown and the apples are bubbling and tender.
Serve:
Let the apple crumble cool for 10-15 minutes before serving. It’s best enjoyed warm, with a scoop of vanilla ice cream or a dollop of whipped cream.