Preheat the oven to 180C fan.
Begin by washing the plums, cut in half and remove the stone, then cut each in half again. Place into an oven proof dish, sprinkle with the caster sugar and the freshly ground cardamom.
Tip the flour and oats into a medium sized bowl, add the salt and cinnamon. Cut the butter into dice and add to the oat mixture. Rub together with your fingers until it resembles course breadcrumbs. Now add the Demerara sugar and sunflower seeds, cobnuts or hazelnuts, stir to incorporate. Spoon the crumble topping onto the plums then place into the oven and bake for 30 minutes or until golden and the fruit juices start bubbling at the edges of the dish. Serve with cream, Greek yogurt or ice cream.