Prepare the Ragu: Heat the olive oil in a large frying pan over medium-high heat. Add the beef mince and cook, breaking it up with a spoon, until it’s beautifully browned all over with no pink remaining. This browning creates a rich base flavour, giving off a savoury aroma. Drain any excess fat. Add the chopped onion, carrots, and celery to the pan. Sauté for about 8-10 minutes until the vegetables have softened and the onion is translucent, you'll notice a sweet, aromatic smell filling your kitchen. Stir in the minced garlic and cook for another minute until fragrant.

Simmer the Ragu: Pour in the chopped tomatoes, beef stock, tomato purée, and dried oregano. Stir everything together. Bring the mixture to a gentle simmer, then reduce the heat to low, cover, and let it cook for at least 45 minutes to an hour, or even longer if you have time. The longer it simmers, the deeper the flavours will become; you'll see the sauce thicken and darken to a rich, glossy red, and the aroma will be wonderfully complex. Season with salt and pepper to taste.

Make the Béchamel Sauce: While the ragu simmers, melt the butter in a medium saucepan over a medium heat. Add the plain flour and whisk constantly for 2 minutes to create a smooth roux; it should smell slightly nutty and turn a pale golden colour. Gradually whisk in the warmed milk, a little at a time, ensuring no lumps form. Continue to whisk until the sauce thickens to a smooth, creamy consistency that coats the back of a spoon. It will be bubbling gently, and you'll hear the soft plop of the sauce. Season with salt, pepper, and freshly grated nutmeg. Remove from the heat.

Preheat and Prepare Lasagna Sheets: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Bring a large pot of salted water to a rolling boil. Carefully add the lasagna sheets, a few at a time, and cook according to package instructions until al dente – usually 2-3 minutes. They should be pliable but still have a slight bite. Immediately transfer them to a bowl of cold water to prevent sticking, then lay them flat on a clean kitchen towel to drain.

Assemble the Lasagna: Spread a thin layer of béchamel sauce on the bottom of a 20x30cm baking dish. Arrange a single layer of cooked lasagna sheets over the béchamel, slightly overlapping if necessary. Spoon over a generous layer of the beef ragu, then sprinkle with some grated mozzarella and a little Parmesan. Repeat these layers: béchamel, pasta, ragu, mozzarella, Parmesan, until all ingredients are used, finishing with a final layer of pasta, a good amount of béchamel, and a generous topping of both mozzarella and Parmesan. The top should look invitingly white and cheesy.

Bake and Rest: Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and bubbling beautifully. You'll hear a gentle sizzling sound and see the cheese melted and lightly caramelised, smelling absolutely divine. Remove from the oven and, this is crucial for presentation, let it rest for at least 15-20 minutes before slicing. This allows the layers to set, ensuring clean, elegant slices that hold their shape.