Preheat the oven to 180°C, fan 160°C, gas 4. Heat the oil in a 2.5 litre ovenproof casserole dish and sear the lamb in batches (set seared batches aside on a plate covered with greasepoof paper) until browned all over.
Add the onion and garlic to the dish and cook gently for 4-5 mins until softened. Stir in the swede and carrots and toss well with the onion and garlic for a minute. Stir in the tomato purée, flour and thyme then return the lamb to the casserole.
Pour over the stock and bring to the boil. Remove from the heat and arrange the potato slices in a circular pattern to completely cover the top of the meat and vegetable mixture. Dot all over with small pieces of the butter.
Place in the oven, uncovered, and cook for 50-60 mins until the meat and vegetables are very tender and the potato topping is golden and crispy on the edges.
For toddlers, serve with a green vegetable, such as broccoli, on the side. To freeze, leave the cooked filling to cool completely, then cover with cling film then foil. Freeze for up to 3 months. Defrost (overnight in the fridge is best), top with potato slices and butter then continue from step 4.