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Heat oven to 180C/160C fan/gas 4. Put the blueberries, cornflour and sugar in a pan and heat for 5 mins until the berries have defrosted and their juices are starting to simmer and thicken. Stir in the raspberries, bring to a gentle simmer, then tip the fruit into a baking dish, about 20 x 25cm.
For the topping, mix the flour, oats, sugar and a pinch of salt in a large bowl. Rub in the peanut butter using your fingers until the mixture looks like fine crumbs. Pour the melted butter over, and work everything to a lumpy mix with your fingers, ensuring there are no dry crumbs left. Scatter over the fruit, then bake for 25-30 mins until golden and crisp and the fruit is bubbling at the edges. Leave for 5 mins before serving with pouring or clotted cream.