Preheat the oven to 180°C / 160°C fan / gas mark 4.
Place the sliced apples in an ovenproof dish.
Add the caster sugar, cinnamon, and lemon juice, then mix gently to coat the apples.
In a separate bowl, rub the gluten-free plain flour and dairy-free butter together with your fingertips until the mixture resembles breadcrumbs.
Stir in the light brown sugar and gluten-free oats.
Spoon the crumble topping evenly over the apples.
Bake for 30–35 minutes until the topping is golden and the apples are soft.
Leave to stand for a few minutes before serving.