Heat the oil in a pot and sauté the onion until it softens. Add the cabbage and celery and stir for a few seconds. Then add the spinach, tomatoes, green pepper, salt, and black pepper. Stir for 3 minutes.

Pour in the broth and bring to a boil over high heat. Cover the pot, reduce the heat to low, and let it simmer for 15 minutes, or until the vegetables are tender.

Carefully transfer the soup to a blender. Blend for a few seconds until the ingredients are mixed and the soup is smooth. Check the thickness and add water if needed to reach your desired consistency.

Return the soup to the pot and let it boil for 2 more minutes. Serve hot.