Put into a bowl: 75 g ground hazelnuts, 75 g flour, 100 g butter and 100 g brown sugar.
2: Mix with your finger tips until the texture is like coarse sand. It doesn't matter if there are a few bigger lumps.
3:
The plums
Generously butter an oven-proof dish. Preheat the oven to 390°F (200°C).
4: Rinse and dry 800 g plums. Cut in half and discard the stones. Spread the plum halves in the dish. If the fruit are under-ripe, sprinkle with 1 tablespoon brown sugar.
5: Spread the crumble topping over in an even layer, though it doesn't matter if the plums poke through here and there (as in the photo).
6: Bake for 30 to 40 minutes, keeping an eye on the how the top is browning.
7: Enjoy warm or cold.
Remarks
This recipe can be made with all types of plums, whether in season, or ones you have frozen. If you don't have ground hazelnuts, use ground almonds instead. Similarly, you can use caster sugar instead of brown.