Preheat the oven to 180C (fan).
Cut the strawberries in half, then add them to a bowl along with the cornflour, sugar, vanilla, and lemon juice. Stir well to coat the strawberries.
Tip them into an oven proof dish.
In a mixing bowl add the plain flour and butter, then use your fingers to rub it together until it resembles chunky breadcrumbs.
Add in the caster sugar and Demerara, then stir to combine.
Spread the crumble topping over the strawberries and press it down slightly.
Bake it for 40 minutes.
Take it out from the oven and leave it to stand for at least ten minutes before serving with warm custard.
