Heat the oven to 190C/170 fan/gas 5. Toss the apples with 2 tbsp golden caster sugar and put in a 23cm round baking dish at least 5cm deep, or a 20cm square dish. Flatten down with your hand.
For the crumble topping, mix the flour and sugar in a bowl with a good pinch of salt. Rub the dairy-free spread in with your fingertips until the mixture looks like moist breadcrumbs. Shake the bowl and any big bits will come to the surface – rub these in too.
Pour the crumble mix over the apples to form a pile in the centre, then use a fork to even out. Gently press the surface with the back of the fork so the crumble holds together and goes crisp, then lightly drag the fork over the top for a decorative finish. Sprinkle the oats and demerara sugar over evenly.
Place on a baking tray and put in the oven for 35-40 mins, until the top is golden and the apples feel very soft when you insert a small, sharp knife. Leave to cool for 10 mins before serving.