Heat the olive oil in a large, heavy-based saucepan or casserole dish over a medium-high heat. Add the lamb mince and cook for 8-10 minutes, breaking it up with a wooden spoon, until it's deeply browned all over. Don't rush this step; the browned bits create flavour. Drain off any excess fat.

Add the chopped onion, carrots, and celery to the pan with the mince. Reduce the heat to medium and cook for 10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. Stir in the crushed garlic and cook for one more minute until fragrant.

Sprinkle over the plain flour and stir well to coat everything. Cook for 1-2 minutes, then stir in the tomato purée and Worcestershire sauce.

Gradually pour in the hot stock, stirring constantly to avoid lumps. Add the thyme and the bay leaf, then bring the mixture to a gentle simmer. Reduce the heat to low, cover with a lid, and let it simmer gently for 30 minutes, stirring once or twice. The sauce should be rich and thickened.

While the filling simmers, cook the potatoes. Place the chopped potatoes in a large pan of cold, salted water. Bring to the boil and cook for 15-20 minutes, or until very tender when pierced with a knife.

Drain the potatoes well in a colander and let them steam dry for a couple of minutes. Return them to the warm pan and mash thoroughly until smooth. What works best for me is using a potato ricer for the smoothest texture.

Add the butter, warmed milk, half of the grated cheese, and a pinch of nutmeg to the mash. Beat well until creamy and smooth. Season generously with salt and pepper to taste.

Preheat your oven to 200°C (180°C fan). Remove the bay leaf from the lamb filling and season it with salt and pepper. Spoon the filling into a 2-litre ovenproof dish and level the surface.

Carefully spoon the mashed potato over the filling, spreading it to the edges to create a seal. Use a fork to fluff up the surface of the mash, creating ridges that will become wonderfully crisp.

Sprinkle the remaining grated Cheddar cheese over the top. Place the dish on a baking tray to catch any drips and bake for 25-30 minutes, until the topping is golden brown and the filling is bubbling up at the sides. Let it stand for 10 minutes before serving.