Arrange the apples, cut side up, in a shallow ovenproof dish. Divide the raisins among the apples, pressing them into the core cavity. Drizzle with 2 tablespoons of the golden syrup and add the apple juice or water to the base of the dish.
Put the flour, oats, sugar and butter into a small bowl and rub in the butter with fingertips until the mixture resembles fine breadcrumbs. Stir in the seeds. Spoon the crumble over the top of the apples and mound up. Drizzle with the remaining syrup.
Cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes until the crumble is golden and the apples are soft. Serve warm with scoops of vanilla ice cream or crème fraîche.