Heat the slow cooker if necessary. Heat the oil in a large frying pan. Tip the onions and thyme sprigs and fry for 2-3 mins. Then add the carrots and fry together, stirring occasionally until the vegetables start to brown. Stir in the mince and fry for 1-2 mins until no longer pink. Stir in the flour then cook for another 1-2 mins. Stir in the tomato purée and lentils and season with pepper and the Worcestershire sauce, adding a splash of water if you think the mixture is too dry. Scrape everything into the slow cooker.
Meanwhile cook both lots of potatoes in simmering water for 12-13 minutes or until they are cooked through. Drain well and then mash with the crème fraîche. Spoon this on top of the mince mixture and cook on Low for 5 hours - the mixture should be bubbling at the sides when it is ready. Crisp up the potato topping under the grill if you like.