Place the onion, garlic and bacon in a large saucepan over a medium heat and fry for 2–3 minutes until lightly golden. Add the sweet potatoes, parsnip, thyme and stock and bring to the boil, then reduce the heat, cover and simmer gently for 15 minutes.
Transfer two-thirds of the soup to a blender or food processor and blend until smooth.
Return the blended soup to the pan, add the cabbage and simmer for a further 5–7 minutes until the cabbage is just cooked. Season with salt and pepper and serve hot.