Heat 3 tbsp oil in a large pan and fry onions until soft.
Add carrots, celery, spices and garlic and cook, stirring, 5 mins more. Stirring often.
Add the lentils, sweet potato and stock, stir well.
Cover and cook for around 30 mins until the carrots and sweet potato are tender, stirring the bottom of the pan with wooden spoon to prevent the lentils from sticking.