In a large saucepan, heat the olive oil and add the onion, carrot, celery, and salt. Cook for 3 minutes.
Add the potato, zucchini, and green beans, and cook for another 3 minutes.
Then add the tomatoes and the 4 cups of chicken broth. Season with salt, pepper, and oregano.
Bring to a boil and cover.
Cook over medium heat for 15 minutes.
Add the cannellini beans and baby spinach, then simmer for 10 minutes, or until all the vegetables are tender.
Season with additional salt if needed and sprinkle with grated Parmesan cheese.