Roast and Peel the Hazelnuts: Preheat your oven to 350°F (180°C). Spread the hazelnuts on a baking sheet in a single layer and roast for 8-10 minutes, or until their skins crack. Remove from the oven and wrap the warm hazelnuts in a clean kitchen towel. Rub them together inside the towel to remove most of the skins (it's fine if some bits remain). Coarsely chop the peeled hazelnuts and set aside.
Prepare the Filling: In a large bowl, toss the sliced plums with the flour and cinnamon until evenly coated. Transfer the mixture to a 20 x 20 cm baking dish, spreading it out evenly.
Make the Hazelnut Crumble: In a medium bowl, mix together the flour, sugar, and a pinch of salt. Add the cold, cubed butter and use your fingers to rub it into the dry ingredients until it forms a coarse, crumbly texture. Stir in the chopped hazelnuts.
Assemble and Bake: Evenly sprinkle the crumble mixture over the plums in the baking dish. Bake for 35-40 minutes, or until the topping is golden brown and the plum filling is bubbling. Allow the crumble to cool slightly before serving.
Food processor method to make crumble topping: For an even quicker method, you can make the crumble topping in a food processor. Simply place the flour, sugar, and salt in the food processor. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs. Once it's ready, transfer the crumble to a bowl and stir in the chopped roasted hazelnuts. This method ensures a perfect crumbly texture with minimal effort!
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