In a large stockpot, heat the oil over medium heat. Add the onion and garlic and saute for 5-6 minutes, until the onion is softened. Stir in 2 tablespoons 9-Spice Mix and cook for a minute or so, until fragrant.
Add the cabbage and diced tomatoes with their juices. Simmer over medium to high heat for 5 minutes.
Add the broth, red lentils, and sweet potato. Stir. Cover and simmer over medium heat for 15 to 20 minutes, or until the lentils and sweet potato are tender.
Season with salt and pepper to taste, or add more 9-Spice Mix. Serve with a dollop of Cashew Sour Cream, if using (for a non-vegan option, use plain yogurt or sour cream). The soup keeps in the fridge for a week, and freezes well for 1-2 months.