Start by preparing all your vegetables. Chop the cabbage, onion, carrots, celery, bell pepper, and mince the garlic.
Heat the olive oil in a large pot over medium heat. Add the onions, carrots, and celery, and sauté for about 5 minutes until softened.
Stir in the bell pepper and garlic. Cook for an additional 2 minutes until fragrant.
Add the chopped cabbage to the pot, mixing well with the other vegetables.
Pour in the vegetable broth and add the diced tomatoes, thyme, and black pepper. Stir everything well.
Bring the soup to a boil, then reduce the heat to low and let it simmer for about 25 minutes, or until the cabbage is tender.
Taste the soup and add salt as needed before serving.
Serve hot, garnished with fresh parsley if desired.