1.2kg/2lb 10¼oz lamb neck fillets, each butterflied, then beaten out to 15cm x 30cm x 2cm/6in x 12in x 1in rectangles using a meat mallet
Preheat the oven to 220C/430F/Gas 7.
Heat the oil in a saucepan over a medium heat and fry the garlic and rosemary for 2-3 minutes. Once the garlic is pale golden-brown, add the drained beans. Continue to heat, stirring regularly, for 10-12 minutes, or until the beans begin to break down.
Stir in the hazelnuts and season, to taste, with salt and freshly ground black pepper. Mash the mixture lightly, using a fork, then spoon onto a plate and set aside to cool.
Blanch the cavolo nero in a pan of boiling, salted water for four minutes. Drain well and, when cool enough to handle, gently squeeze to remove any excess water.
Lay each flattened rectangle of neck fillet onto a large piece of cling film and season, to taste, with salt and freshly ground black pepper. Arrange a layer of cavolo nero leaves onto each neck fillet and divide the bean mixture between them. Wet your hands and, using the flats of your palms, press the mixture down to form a compact, even layer.
Starting with the short edge of the rectangle of lamb, gently but firmly roll up the lamb, using the cling film to keep the shape tight and even, until the lamb forms a neat roll.
Remove the cling film and tie 3-4 pieces of string around each lamb fillet to hold it in place. (NB: If making in advance, cover the rolled lamb with cling film and store in the fridge overnight. Remove from the fridge 3-4 hours before cooking.)
Place the stuffed lamb fillets onto a lightly oiled baking tray and season, to taste, with salt and freshly ground black pepper. Transfer to the oven and cook on the top shelf for 20-25 minutes (for medium), or until cooked to your liking. Once cooked, remove from the oven, cover with aluminium foil and set aside to rest for at least 10 minutes.
To serve, carve the rolled, stuffed lamb into slices and divide the slices equally among six serving plates.