1. Stone the plums, and poach until just tender with a couple of tablespoons of water.
2. Spoon into an oven-proof dish brushed with hazelnut oil.
3. Mix the sugar, flour and hazelnuts, and then gently work in the butter, rubbing and lifting until the mixture resembles breadcrumbs. Whole hazelnuts that you grind yourself to an uneven texture are also very good in a crumble.
4. Spoon the crumble topping over the fruit and bake in a pre-heated oven at 180 C, gas mark 4 for 20-25 minutes, until the top is golden brown.