Put the plums, blackberries, sugar and star annise in a large saucepan with a splash of water. Simmer gently over a medium heat for 15-20 minutes, until softened. Add a splash more water if needed.
Meanwhile, put 1 tbsp of cornflour in a small bowl and mix with a little water until the mixture resembles a thick paste and the cornflour is completely dissolved. Add the cornflour mixture to the saucepan with the fruit and stir until combined. Keep warm until serving.
To make the crumble topping, melt the butter in a small pan, then remove from the heat. Put the granola and chopped hazelnuts in a bowl and pour over the butter. Give everything a good stir, then drizzle over the honey and mix again until all the ingredients are combined.
Remove the fruit mixture from the heat and divide between four ramekins. Top the fruit with 1-2 tablespoons of the crumble topping. Serve warm, with vanilla custard or ice cream on the side, if you like.