Preheat the oven to 180°C/ Fan 160°C/ Gas 4. Mist a medium baking dish with cooking spray.
Put the plums, blackberries, flour, lemon zest and cardamom in a large mixing bowl and toss gently to evenly coat the fruit. Spoon the mixture into the prepared baking dish.
Combine the oats, sugar, butter and a pinch of salt in a medium bowl. Using your fingertips, rub everything together until the mixture clumps, then stir in the pistachios.
Scatter the crumble mixture evenly over the fruit, then bake for 30-35 minutes, until the crumble is golden and the fruit filling is bubbling. Set aside to cool for 15 minutes before serving with cream, ice cream or yoghurt on the side.