Preheat the oven to 200C/400F/Gas 6.
Season the lamb with salt and freshly ground black pepper. Heat a frying pan until smoking, then add half of the butter to the pan. When the butter is foaming, add the lamb rumps and sear on all sides for 2-3 minutes, or until golden-brown all over. Remove from the pan and set aside.
Place the hazelnuts and rosemary into a food processor and pulse until the mixture resembles fine breadcrumbs. Tip the mixture out onto a plate. Brush the lamb evenly all over with the mustard, then roll in the hazelnuts to coat all over.
Place the lamb back into the frying pan and place into the oven for 12-15 minutes (for medium), or until cooked to your liking. Remove the lamb and set aside to rest for five minutes.
Heat the remaining butter in a clean frying pan until foaming, then add the runner beans to the pan. Pour in enough water to just cover the beans, then bring to a simmer and cook for 3-4 minutes, or until the beans are just tender and most of the liquid has evaporated. Season, to taste, with salt and freshly ground black pepper and stir in the chopped mint and chives.
To serve, carve the lamb into thick slices. Spoon the mashed potato into the centre of each serving plate and top each mound with a few slices of the lamb. Place the runner beans alongside, then drizzle over any of the pan juices from cooking the lamb.