Heat olive oil over medium heat in a large soup pot.
Add onions and celery and saute for 5 minutes until onions are soft and translucent.
Add garlic and continue to cook for one minute.
Add tomato paste, Italian seasoning, thyme (if using) and bay leaf and let cook for another minute.
Add broth/water, canned diced tomatoes with juice, mushrooms, and barley to the pot. Turn the heat to medium-high, cover the pot, and bring to a boil.
Once the contents have come to a boil, reduce the heat to maintain a gentle simmer (still covered). Simmer for 35 minutes, until barley is tender
Stir in the cannellini beans, fresh carrots, beans, and corn, and cook for 5 more minutes.
Remove bay leaf and adjust salt and pepper to taste. Garnish individual bowls with chopped parsley.