Cook cabbage in 1.5 litres boiling salted water for about 10 minutes. Drain, reserving the water. Rinse cabbage in cold water then roughly chop into smaller, bite-sized pieces.
Heat olive oil in a large saucepan and cook onion, garlic, pancetta and salt over moderately high heat until onion is soft and pancetta lightly browned, about 3 minutes. Add rosemary leaves and cook for 2 minutes. Break up chestnuts a little and add to pan with the cabbage. Stir to infuse flavours, then add wine. Bring to boil and cook for 1 minute. Add cabbage water and simmer for about 15 minutes.
Remove half the soup from the pan and purée, then return to the pan.
Gently reheat and check seasoning.
Divide soup among four large bowls. Drizzle with some oil, then add a good grind of black pepper. Serve with crusty bread.