Preheat the oven to 180 C / 375 F
Remove the stones from the greengages by slicing them in half along the natural seam/indentation you'll see on their skin. Then, using a sharp knife, carefully cut around the stone to get it out. Don't worry too much about them looking neat...they are all going to cook into a pile of warm, sticky fruity mess under your crumble anyway.
Place them in the bottom of a buttered ovenproof dish, skin side down.
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.
Mix in the nuts and spread over the top of the greengages. Sprinkle over some cinnamon and a bit of extra sugar and bake for 30 minutes. 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.