Prepare the Potatoes: Preheat your oven to 180°C (160°C fan). Place the sliced potatoes in a large saucepan of salted water. Bring to the boil and cook for 5-6 minutes until they are just starting to become tender but still hold their shape. Drain carefully and set aside.

Sauté the Aromatics: While the potatoes are boiling, heat the olive oil in a large, deep frying pan or Dutch oven over a medium heat. Add the chopped onion and cook for 6-8 minutes, stirring occasionally, until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Brown the Beef Mince: Increase the heat to medium-high, add the beef mince to the pan, and break it up with a wooden spoon. Cook for 7-10 minutes, stirring frequently, until the mince is browned all over. It's important to get a good colour on the beef as this is where a lot of the flavour comes from. Be sure to check the Food Standards Agency guidelines for cooking meat safely.

Create the Sauce: Sprinkle the plain flour over the browned mince and stir well to combine. Cook for one minute, then gradually pour in the hot beef stock, stirring constantly to avoid lumps. Add the tomato purée, Worcestershire sauce, dried thyme, and the bay leaf.

Simmer the Filling: Bring the mixture to a gentle simmer. Reduce the heat to low, cover, and let it bubble away gently for 10-15 minutes. This allows the sauce to thicken and the flavours to meld beautifully. Afterwards, remove the bay leaf and season generously with salt and pepper to your taste. What works best for me is tasting it at this stage to ensure the seasoning is just right before assembling.

Assemble the Casserole: Spoon the beef mixture into the bottom of a 2-litre casserole dish and spread it into an even layer. Arrange the par-boiled potato slices on top of the beef, overlapping them slightly to create a tight seal.

Add the Topping and Bake: Brush the melted butter evenly over the top of the potatoes. This will help them turn golden and crisp. Finally, sprinkle the grated Cheddar cheese all over the top.

Bake to Perfection: Place the casserole dish in the preheated oven and bake for 30-35 minutes, or until the potato topping is golden brown and the cheese is gloriously melty and bubbling at the edges.

Rest Before Serving: Once baked, carefully remove the casserole from the oven and let it stand for 10 minutes before serving. This allows the filling to set slightly, making it much easier to serve.