Preheat the oven to 200ºC/ 400ºF/ Gas 6 and lightly oil an ovenproof dish.
Core and slice the unpeeled apples and dress with lemon juice to prevent browning. Place in a saucepan with the blackberries over a low heat and bring to a simmer, cover and simmer for 5 minutes. Turn the fruit into the dish.
Mix together the flour and oats, then rub in the fat until the mixture resembles breadcrumbs. Stir in the dates and sunflower seeds, then sprinkle the mixture over the fruit.
Bake in the oven for 25 minutes