Preheat oven to 350°F (175°C). Grease a 9-inch baking dish.
In a large bowl, toss apple slices with lemon juice, sugar, cinnamon, and nutmeg until evenly coated.
Spread the apple mixture evenly into the prepared baking dish.
In another bowl, combine flour, oats, brown sugar, and salt.
Add cold butter cubes and cut into the mixture using a pastry cutter or your fingers until it resembles coarse crumbs.
Sprinkle the crumble topping evenly over the apple filling.
Bake for 40–45 minutes, until the topping is golden brown and the apples are tender.
Let cool slightly before serving. Best enjoyed warm with ice cream or custard.
