Heat the oil in a large saucepan, add the onion and fry for 5 minutes, stirring until softened. Stir in the garlic, curry paste and ginger and cook for 1 minute.
Mix in the potatoes, carrots, cauliflower and lentils. Pour in the stock and tomatoes, season with salt and pepper and bring to the boil. Cover and simmer for 30 minutes or until the lentils are tender.
Meanwhile, mix the yogurt, coriander and mango chutney together to make the raita and spoon into a small bowl.
Add the spinach to the soup and cook for 2 minutes until just wilted. Taste and adjust the seasoning if needed. Ladle the soup into shallow bowls, top with spoonfuls of raita. Serve with warmed naan breads, if liked.