Preheat the oven to 220°C.
Fry garlic and rosemary in oil for 3 minutes, then add beans and heat until breaking down.
Stir in hazelnuts, season, mash lightly with a fork, and let cool on a plate.
Blanch cavolo nero in boiling salted water for 4 minutes and squeeze out excess water.
Lay lamb onto cling film, season, and layer with cavolo nero and the bean mixture.
Roll the lamb tightly using the cling film to maintain a neat shape.
Remove film and tie the lamb fillets with string to secure them.
Place lamb on an oiled tray and roast at 220°C for 25 minutes before resting.
Carve the lamb into slices and serve across six plates.