Heat the extra-virgin olive oil in a large pot over medium heat. Add the chopped onion and sage leaves. Sauté for 3-5 minutes until the onion is tender and translucent. Then add the minced garlic and sauté for a minute.
Add the chopped carrot and celery and cook for another 5 minutes. This will help to develop the typical flavor of an Italian soffritto.
Drain and rinse your cannellini beans well; add them to the pot with 4 cups of vegetable broth and the rosemary sprigs. Bring to a boil, then reduce the heat to a gentle simmer and cook for 8-10 minutes.
Discard the rosemary sprigs, transfer half of the soup to a high-speed blender, and blend until creamy and smooth.
Mix the blended soup with the rest of the soup in the pot, and if needed, add more broth.
Add the cavolo nero and simmer on low heat for another 5-8 minutes until the kale wilt and becomes tender (see note 2).
If the soup is too thick for your liking, add more broth to thin it down. Adjust salt and pepper to taste.
Remove from the heat, add the chopped parsley, mix well, and serve hot with some crusty bread. If you like a bit of heat, sprinkle some chili flakes on top.