In a large stockpot, heat the oil over medium heat. Add the sausage and cook until no longer pink and just turning brown, 5 to 8 minutes. Add the celery, carrots and onions and saute until translucent, about 15 minutes. Add the rosemary, tomato paste and garlic and cook for 1 minute. Stir in the stock, tomatoes, beans and bay leaf and bring to a simmer. Simmer for 40 minutes.
Add the sweet potatoes and zucchini and simmer until cooked al dente, about 10 minutes more.
Place 1/2 cup of baby spinach at the bottom of each serving bowl and ladle on a portion of bubbling hot soup. The heat from the soup will wilt the spinach perfectly. Serve with some freshly grated Parmigiano-Reggiano and a squeeze of fresh lemon.