While the lamb is resting, melt the butter in a large frying pan over a medium heat. Once bubbling, add the anchovies and cook for 2 mins until they melt away. Stir in the garlic, fry for 1 min more, then toss in the greens and cook for 8 mins until wilted.
Add the lemon juice and parmesan and stir well to melt the cheese. Season to taste, transfer to a serving bowl and scatter over the hazelnuts.