First, make the crumble topping. Put the flour and butter in a food processor and pulse to the texture of wet sand. Tip into a bowl and add the sugar and oats. Sprinkle the mixture with 2 tbsp cold water, then use a fork to mix everything together, creating a crumbly texture with a few large clumps. Chill until needed.
Heat oven to 200C/180C fan/gas 6. Tip all the fruit ingredients, except the blackberries, into a large ovenproof dish. Add 50ml water, cover with foil and bake for 1 hr. Remove the foil and discard star anise. Add the blackberries to the dish, sprinkle over the crumble mixture and return to the oven for a further 25-30 mins until the top is golden. Serve hot with custard.