Heat the olive oil in a large pot over medium heat.
Add the onion and garlic and sauté until translucent, about 3 minutes.
Stir in the carrots and celery, and cook for another 4-5 minutes.
Add the chopped cabbage, zucchini, diced tomatoes, and vegetable broth.
Season with Italian seasoning, salt, pepper, and red pepper flakes if using.
Bring to a boil, then reduce the heat and simmer uncovered for 25–30 minutes, or until vegetables are tender.
Stir in the lemon juice.
Serve hot, garnished with fresh chopped parsley.