Preheat the oven to 170°C.
Put the frozen fruits in a baking dish, then add half of the cinnamon, the vanilla and 4 spoonful of the sugar (add more if you have a sweet tooth). Add the nuts too, if used.
Mix the oats, sugar, remaining cinnamon and flour, then add dots of the butter or spread. Work them together with your fingers, but not too finely, then scatter these on top the fruits. (You can add some ground nuts too to the mixture, if liked.)
Put the dish in the middle of the oven and bake for 25-30 minutes, until the topping is golden and the fruits are bubbling.
Serve warm with custard, ice cream or crème fraîche/ vanilla yoghurt