Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until lightly browned. Add the garlic, fennel seeds and ginger and cook for 2 minutes. Add the stock and salt and pepper and bring to the boil.
Put the sweet potatoes and lentils in the slow cooker pot, pour over the hot stock mixture, cover with the lid and cook on low for 6–8 hours or until the potatoes and lentils are soft.
Purée the soup, in batches if necessary, and return it to the slow cooker. Stir in the milk and cook on high for 15 minutes.
Meanwhile, make the garnish. Heat the oil in a clean frying pan, add the onion and fry over a low heat, stirring occasionally, for 10 minutes or until softened. Stir in the spices and sugar, increase the heat slightly and fry for 5 more minutes or until golden brown.
Ladle the soup into bowls and sprinkle the spicy onions over the top. Serve with warm naan bread.