Preheat your oven to 200°C (400°F).
Peel and chop the potatoes into even-sized chunks.
Boil in salted water for about 15 minutes until tender.
Drain and mash with butter and milk until smooth.
While the potatoes are boiling, heat a large pan over medium heat.
Add a splash of olive oil, then add the minced lamb.
Cook until browned, breaking it up with a wooden spoon.
Transfer the mince to a plate and set aside.
In the same pan, add a bit more olive oil if needed.
Finely chop the onions, carrots, and celery.
Add the vegetables to the pan and cook for about 10 minutes until softened.
Return the lamb to the pan with the cooked vegetables.
Stir in tomato purée and Worcestershire sauce, and cook for another 2 minutes.
Pour in the beef stock and bring to a boil.
Reduce the heat and let it simmer for 15 minutes.
Transfer the lamb mixture to a baking dish, spreading it out evenly.
Spoon the mashed potatoes over the top, using a fork to create peaks.
Bake for 20-25 minutes until the top is golden and crispy.
Serve hot and enjoy!