I've still got blackberries from last year, and also found a small container of frozen blueberries while assessing the space in the freezer, prior to this year's gooseberry harvest. I'd also got a couple of Bramley apples to use up, so something using all three seemed a good idea.
Apple and Berry Crumble Cake, which I found on the Good to Know site, was quick to put together, as part of the base mixture was put aside for the crumble topping, rather than needing to make two separate mixtures. I used 300g total weight of mixed blackberries and blueberries as I had less than 100g of blueberries in my little pot in the freezer. I didn't bother thawing the berries ahead of use, either! In place of the mixed nuts in the topping, I used toasted chopped hazelnuts, but apart from these minor changes, I followed the recipe exactly. I didn't need to cover the cake while it was baking - even after the full cooking time it was only lightly browned.
I was worried that the combination of Bramley apples and frozen fruit would make the cake too wet but this wasn't the case, if anything the crumble topping was too dry and powdery. I think this would have made the cake quite unpleasant to eat on it's own, but it was fine as a dessert with vanilla yogurt. The fruit didn't have any extra sugar added to it, so it made a nice sharp layer between the sponge and the crumble.