Make the nut crumble topping by first toasting hazelnuts on a shallow tray in the oven for 10 minutes. Allow them to cool for 5 minutes, and then remove the skin from the hazelnuts
On a chopping board, use a knife to roughly chop the hazelnuts. I like a crunchy, chunky nut crumble so I leave mine pretty big, but this is up to you.
Now make your hazelnut crumble topping by blending the butter and the flour together, and then slowly adding the sugar. Don't blend it for too long as we don't want to overwork it.
You'll know it's done when the greengages are bubbling up through the crumble at the sides and the hazelnut crumble topping is a light warm brown. Serve warm with vanilla ice cream and lashings of cream.