01 - Preheat the oven to 350°F. Lightly grease a 9-inch baking dish.
02 - In a large bowl, combine sliced apples with lemon juice, granulated sugar, cinnamon, and nutmeg. Toss to coat evenly. Transfer the mixture to the prepared baking dish.
03 - In a separate bowl, mix flour, rolled oats, brown sugar, and salt. Cut in the cold butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Stir in nuts if desired.
04 - Evenly distribute the crumble topping over the apple layer in the baking dish.
05 - Bake for 40 minutes or until the topping is golden brown and the apple filling is bubbling.
06 - Allow to cool slightly before serving. Enjoy warm, optionally accompanied by vanilla ice cream or whipped cream.