Place the olive oil in a Dutch oven or large heavy-bottomed pot and cook over medium heat. Add the pancetta, if using, and cook, stirring often, until browned around the edges, about 5 minutes. Add the onion, carrots, celery, garlic and red pepper flakes and season with salt and pepper. Cook until the onion is translucent and carrots are tender, 10 to 12 minutes. Add the tomato paste, stirring to coat the vegetables, about 3 minutes. Add the whole tomatoes with the juices, crushing with your hands as you go and add the parmesan rind. Add 6 cups of water (or broth). Add the potatoes, reduce heat, and simmer until potatoes are tender about 20 to 25 minutes. Add the beans and the kale or spinach and cook until wilted, about 5 minutes. Remove and discard the parmesan rind and add the fresh herbs. Season to taste and add a splash of vinegar.