Simmer stock, beef bones, one onion, carrot, and celery for two hours.
Strain stock through a fine sieve and set aside.
Cook four chopped onions in butter over low heat for 15 minutes.
Deglaze onions thrice using white wine in increments until they are caramelized and soft.
Stir in flour and cook for one minute.
Add 1.5 litres of stock and diced potato, then simmer for 40 minutes.
Cool soup slightly then blend with a hand-held blender until smooth.
Blanch the cavolo nero in the remaining boiling stock.
Pour cabbage and blanching stock into the blended soup.
Mix the cheeses in a bowl and preheat a hot grill.
Toast sliced baguette on both sides.
Ladle soup into bowls, top with baguette and cheese, and grill until bubbling.