Preheat the oven to 190C

Place the potatoes into a large saucepan of water and bring to the boil. Reduce the heat to a simmer, fit with a tightly fitting lid and continue to cook until the potatoes are softened. Strain the potatoes into a large colander and return to the saucepan with the butter. Mash the potatoes until completely smooth. Check the seasoning and keep aside.

Meanwhile, over high heat, drizzle the oil in a large sauté pan, add the lamb mince and sear for a 4-5 minutes until browned, breaking up any lumps with the back of a wooden spoon. Remove from the heat and pour the coloured mince lamb into a bowl.

Place the sauté pan back over high heat. Add the onion, celery, carrot and garlic together with the dried herb and sauté for 3-4 minutes.

Stir in back the minced lamb into the sauté pan and season well with salt and pepper.

Add the tomato puree, Worcestershire sauce and stock and bring to a simmer. Cook gently with a lid for another 10-15 minutes until completely tender. Check the seasoning.

Place the mixture into a gratin or oven safe dish and spoon the potatoes onto the top of the meat mixture.

Using a fork, smooth the potato topping and add some extra dices of fresh butter with some salt and pepper.

Place in the oven to roast for 20-25 minutes until bubbling hot and golden brown. Serve immediately.