Preheat your oven to 180°C (350°F) and lightly grease a baking dish.
Peel, core, and slice the apples evenly, then toss them with lemon juice, cinnamon, half of the brown sugar, and vanilla extract.
In a bowl, combine flour, oats, the rest of the sugar, and salt. Rub in the butter using your fingertips until the mixture resembles coarse crumbs.
Spread the prepared apples evenly in the dish, then sprinkle the crumble mixture over the top.
Bake for 35–40 minutes, or until the topping turns golden brown and the filling bubbles at the edges.
Allow the crumble to rest for 10 minutes before serving warm with custard, vanilla ice cream, or cream.