Warm the olive oil in a large pot over medium heat and add the onion, garlic, celery, and carrots. Cook while stirring occasionally until the veggies are tender and fragrant, about 5 minutes.
Pour in the diced tomatoes, tomato paste, and vegetable broth, then bring everything to a gentle simmer and let it meld for 5 to 10 minutes.
Add the zucchini, green beans, and beans into the pot, mix well, and simmer uncovered for 20 to 30 minutes until all the vegetables are soft.
Season the soup with Italian seasoning, salt, black pepper, and red pepper flakes if using; taste and adjust the seasoning as needed.
Serve the soup hot, topped with Parmesan and parsley if you like, and pair it with crusty bread for a complete meal.