1 1/2 tablespoons extra-virgin olive oil
1 medium leek or yellow onion, finely chopped
2 medium carrots, peeled and finely chopped
2 medium celery stalks, finely chopped
1 tablespoon minced fresh rosemary
2 tablespoons tomato paste
2 large cloves garlic, minced (about 1 tablespoon)
5 cups chicken broth
Two 15-ounce cans cannellini beans, rinsed and drained
6 ounces (about 1 bunch) lacinato kale, center ribs removed, leaves chopped
1 Parmesan rind (1-by-3 inches; optional), plus grated Parmesan for garnish
3/4 pound hot bulk Italian sausage, rolled into bite-size meatballs
2-3 tablespoons cider vinegar, to taste
Freshly ground black pepper
Heat 1 tablespoon of the oil in a 4- to 5-quart pot over medium heat. Add the leek or onion, carrot, celery, rosemary, and a generous couple pinches of salt and cook, stirring occasionally, until the vegetables begin to soften, about 6 minutes. Add the tomato paste and garlic and cook until fragrant, 45 seconds. Add the broth, and parmesan rind (if using). Bring to a boil, reduce the heat to medium low, and simmer gently until the vegetables are tender, about 15 minutes.
Meanwhile, heat the remaining 1/2 tablespoon oil in a 10-inch nonstick skillet over medium-low heat. Add the sausage meatballs, and cook, flipping about halfway through, until browned and cooked through, about 10 minutes.
Add the kale to the soup; cook 1 minute, then add the meatballs (discarding any of the rendered fat and oil) and the beans; cook for 4 more minutes to meld the flavors. Stir the cider vinegar into the soup and season to taste with salt and pepper.
Ladle into bowls and garnish with grated parmesan.