Start the Mash: Place the quartered potatoes in a large saucepan and cover with cold, salted water. Bring to a boil over high heat, then reduce to a simmer. Cook for 15-20 minutes, or until a knife slides through the largest piece with absolutely no resistance. The smell should be earthy and clean. Drain the potatoes thoroughly in a colander, then return them to the dry pan over a low heat for a minute to steam off any excess moisture. This is crucial for a fluffy mash.

Build the Vegetable Base: While the potatoes boil, heat the sunflower oil in a large, heavy-based frying pan or casserole dish over a medium heat. Add the onion, carrots, and celery. Cook gently for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. You will hear a gentle sizzle, and the kitchen will fill with the sweet smell of cooking onions. Add the minced garlic and cook for a further minute until fragrant.

Brown the Beef: Turn the heat up to medium-high. Add the ground beef to the pan, breaking it up with a wooden spoon. Cook for 5-7 minutes, stirring, until the meat is no longer pink and has taken on a deep, brown colour. You should hear a vigorous sizzle; this is the sound of browning, not steaming. If the pan seems too wet, cook for another minute to evaporate the liquid.

Create the Gravy: Stir in the tomato purée and cook for 2 minutes, stirring constantly. It will darken in colour and smell intensely rich. Sprinkle over the dried thyme, then pour in the Worcestershire sauce and the hot beef stock. Stir everything together, scraping the bottom of the pan to release any delicious browned bits (the fond). Bring the mixture to a gentle simmer. Let it bubble away for 10 minutes, or until the liquid has reduced by about a third and the sauce has thickened. The filling should look glossy and rich, not watery.

Finish the Mash: While the filling simmers, mash the drained potatoes with a potato masher or ricer until smooth. Add the butter and a generous pinch of salt and pepper. Begin to beat in the warm milk, a little at a time, until you have a creamy, spoonable consistency. Taste and adjust seasoning. The texture should be light and airy, not dense or sticky.

Assemble and Bake: Preheat your oven to 200°C (400°F). Pour the beef filling into a 2-litre (8-cup) baking dish, spreading it into an even layer. Spoon the mashed potato on top, starting at the edges to seal in the filling, then working your way to the centre. Use a fork to create peaks and swirls across the surface—these will catch the heat and become beautifully crisp. If using, scatter the grated cheddar over the top.

The Golden Finish: Place the dish on a baking tray to catch any spills. Bake for 25 minutes, until the filling is bubbling up around the edges and the potato is starting to colour. For the final 5 minutes, switch the oven to a high grill (broil) and watch closely. The peaks of potato will blister and turn a deep, golden brown. The sound of it bubbling and the sight of that crust forming is the ultimate signal that it is ready. Let it rest on the counter for 10 minutes before serving.