There are many ways to make a delicious soup. The success of a soup is in the depth of flavors. The first layer of flavor is the pancetta, cured and salted pork belly. Olive oil is a good substitute if you don’t have or want to use pancetta. Another layer of flavor are the aromatics, vegetables used as a base flavor including, but not limited to, celery, carrots, onions and fresh herbs. Adding a piece of the rind from a block of parmesan cheese adds a tremendous amount of richness, without the addition of fat. The beans, tomato paste, and kale are all a bonus. Kosher or sea salt and freshly ground pepper are a must, but gradually season the soup throughout the cooking process. Taste it, then taste it again and adjust the seasoning. Keep in mind that both the pancetta and cheese rind add salt. A splash of sherry vinegar for the finish helps balance all the flavors. The last layer of flavor comes from a pistou, which is a combination of garlic, basil, parmesan cheese, lemon zest and olive oil, it’s like a pesto sauce, but without the pine nuts.
Place the oil in a Dutch oven or large heavy-bottomed pot and cook over medium heat. Add pancetta, if using, and cook, stirring often, until browned around the edges, about 5 minutes. Add 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 the parmesan rind, if using. Add 6 cups of water (or broth). Add potatoes, reduce heat, and simmer until potatoes are tender about 20 to 25 minutes. Add the beans and the kale and cook until kale is wilted, about 5 minutes. Discard parmesan rind and add the fresh herbs. Season to taste, and add the splash of vinegar.
To make the pistou: Pulse the basil and the garlic in a food processor until finely chopped. Transfer to a small bowl and stir in olive oil, parmesan cheese and lemon zest, and season with salt. Serve the soup topped with the pistou.