Prepare the Potatoes: First, preheat your oven to 180°C (160°C fan). Place the sliced potatoes in a large pan of salted water, bring to the boil, and cook for 5-7 minutes until they are just starting to become tender but still hold their shape. Drain them carefully and set aside.

Cook the Mince Base: While the potatoes are boiling, heat the olive oil in a large, deep frying pan or casserole dish over a medium-high heat. Add the chopped onion and cook for 5 minutes until softened. Add the minced garlic and cook for another minute until fragrant.

Brown the Beef: Add the lean beef mince to the pan, breaking it up with a wooden spoon. Cook for 8-10 minutes, stirring occasionally, until the meat is thoroughly browned all over. Drain off any excess fat if necessary.

Create the Beef Sauce: Stir the 2 tablespoons of plain flour into the mince and cook for a minute. Gradually pour in the beef stock, stirring constantly to avoid lumps. Add the tomato purée, Worcestershire sauce, and mixed herbs. Season with salt and pepper, bring to a simmer, then reduce the heat and let it bubble gently for 10 minutes to thicken.

Make the Cheese Sauce: In a separate saucepan, melt the 50g of butter over a medium heat. Stir in the 50g of plain flour to form a paste (a roux). Cook for one minute, then gradually start adding the milk, a little at a time, whisking constantly until you have a smooth, lump-free sauce. Stir in the mustard powder and nutmeg.

Add the Cheese: Remove the sauce from the heat and stir in about 200g of the grated Cheddar. Keep stirring until the cheese has completely melted into the sauce. Season with a little salt and pepper to taste.

Assemble the Casserole: Spoon the beef mince mixture into the bottom of a large ovenproof dish (around 2.5 litres). Arrange the par-boiled potato slices over the top of the mince in an even, slightly overlapping layer. I find that taking a moment to arrange them neatly makes the final dish look fantastic.

Bake to Perfection: Pour the cheese sauce evenly over the potatoes, making sure it gets into all the gaps. Sprinkle the remaining 50g of grated cheese over the top. Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the sauce is bubbling deliciously around the edges.

Rest Before Serving: Let the casserole stand for at least 10 minutes before serving. This allows it to set slightly, making it much easier to serve neat portions.