Heat the olive oil in a Dutch oven over medium heat. Add the onion, carrots, and celery and sauté 5 minutes until softened. Stir in the garlic, potatoes, zucchini, and green beans and cook 3 minutes more.
Add the diced tomatoes, broth, tomato paste, salt, pepper, oregano, basil, and bay leaf. Stir well. If using meat, brown it first in the pot, then add with the broth.
Bring to a boil, then reduce the heat to low and simmer gently 1 hour, stirring occasionally.
Stir in the beans and balsamic vinegar. Simmer 15-20 minutes more, until all the vegetables are tender and the flavors meld.
Remove the bay leaf, adjust the seasoning, and garnish with fresh parsley before serving.