Put the hazelnuts in the bowl of a food processor and pulse until the nuts are finely ground. Add the flour, 125 g / 4.5 oz sugar, and a pinch of salt. Pulse briefly until everything is well combined. Add the cold butter, plus 1 tbsp of cold water and pulse briefly until the mixture resembles breadcrumbs. Tranfer the dough to a bowl and press the crumbs with your fingers into larger lumps if you think the crumbs are too fine. Let rest in de fridge for about 30 minutes.
Preheat the oven to 180°C / 350°F. Line the baking pan with parchment paper with a little overhang (this way you can lift the crumble out easily once cooled).
In a bowl, mix the cranberries with 100 g / 3.5 oz sugar, the cornstarch, and the cinnamon. Zest and juice the clementines or the orange and add the zest and 50 ml / 1.6 fl oz juice to the cranberries. Toss until well combined.
Press half of the dough firmly into the bottom of the prepared baking pan. Spoon the cranberry mixture (with the juices) evenly over the dough. Sprinkle the remaining crumble dough over the cranberries.
Bake in the oven for about 40-45 minutes until the top is golden brown. Let cool completely before cutting it into 12 bars.
Dust with powdered sugar and serve with vanilla ice cream. The bars can be stored in a sealed container for up to 3-4 days.