Heat olive oil in a large pot over medium heat. Add chopped onion and garlic, sautéing until the onion is translucent, about 5 minutes.
Add the bell pepper, carrots, and celery to the pot. Cook for another 5 minutes, stirring occasionally.
Stir in the chopped cabbage, diced tomatoes (with juice), vegetable broth, thyme, basil, salt, black pepper, and red pepper flakes (if using). Bring the mixture to a boil.
Reduce heat to low and let the soup simmer for about 30 minutes, or until the vegetables are tender.
Stir in the apple cider vinegar and cook for an additional 5 minutes.
Taste and adjust seasoning if necessary before serving.