Heat oven to 220C/200C fan/gas 7. Slice the marrow open lengthways and scoop out the seeds. Cut the marrow into slices and place on a baking tray. Toss with the olive oil, garlic, maple syrup and some seasoning. Roast for 20 mins, then sprinkle the hazelnuts and breadcrumbs over. Roast for another 8 mins, then remove.
While the marrow is roasting, prepare the rest of the salad. Shred the cavolo nero leaves into bite-sized pieces. Put on a large platter or shallow bowl and top with the radishes.
Combine the dressing ingredients in a small bowl, adding some salt. Mix well.
Just before the marrow has finished cooking, drizzle the dressing over the cavolo nero. Use your hands to massage it into the cavolo nero for a few mins so that it softens it, then top with the marrow, breadcrumbs and hazelnuts. Sprinkle the Parmesan shavings over and serve.