In a large stockpot or Dutch oven, heat the olive oil over medium heat. Add the chopped onion, carrots, and celery. Cook, stirring occasionally, until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the chopped cabbage and bell peppers to the pot. Stir everything together and cook for about 5 minutes, allowing the cabbage to wilt slightly.

Pour in the diced tomatoes (with their juice) and the vegetable broth. Stir in the dried oregano, basil, salt, black pepper, and red pepper flakes.

Bring the soup to a boil, then reduce the heat to low. Cover the pot and let it simmer for at least 30 minutes, or until all the vegetables are tender.

Taste the soup and adjust the seasoning if necessary. Serve hot, garnished with fresh parsley if desired.