Wash, peel and chop the potatoes. Put them in a pan of cold water. Bring to the boil and cook for 20-25 minutes.
Heat the olive oil in a frying pan over a medium heat. Add the carrots and onions and soften for 5-6 minutes. Increase the heat and add the minced lamb and thyme. Break up the mince and allow to brown and cook through (3-4 minutes).
Pour the beef stock into the pan and stir in the tomato purée and the Worcestershire sauce. Bring to a simmer, then cover partially and allow to cook for 45 minutes.
Once the potatoes have cooked, drain them, and return them to the pan with the milk and butter. Mash the potatoes until smooth and creamy using a masher.
Add the spring onions and season with the salt and pepper.
Preheat the oven to 180°C (Gas 4). Season the lamb with salt and pepper and transfer to an overproof baking dish. Top with the champ mash.
Add some more salt and pepper to the topping and bake for an additional 25 minutes until the top browns and the mince begins to bubble slightly around the edges.