Warm the olive oil in a large pot over medium heat. Add the onion, garlic, celery, and carrots, cooking until the vegetables become tender and release their aroma, about 5 minutes.
Add the diced tomatoes, tomato paste, and vegetable broth to the pot. Bring the mixture to a gentle simmer and allow it to blend together for 5 to 10 minutes.
Incorporate the zucchini, green beans, and kidney beans into the soup. Stir well and let everything cook on a low simmer for 20 to 30 minutes until the vegetables are soft.
Season the soup by mixing in Italian seasoning, salt, black pepper, and red pepper flakes if you like some heat. Adjust the flavors to your liking.
Serve the soup hot, garnished with Parmesan cheese and fresh parsley if desired. Pair with breadsticks or crusty bread for a satisfying meal.