Preheat and Cook Pasta: Preheat your oven to 180°C (160°C fan). Bring a large pot of heavily salted water to a rolling boil. Add the macaroni and cook according to package directions, but for 2 minutes less than the recommended time. It should be very al dente. Drain the pasta and set it aside.

Start the Roux: While the pasta is cooking, melt the 60g of butter in a large, heavy-bottomed saucepan or Dutch oven over a medium heat. Once melted, whisk in the plain flour to form a thick paste (this is the roux). Cook, stirring constantly, for 2 minutes. This cooks out the raw flour taste.

Make the Béchamel Sauce: Gradually pour in the warm milk, a little at a time, whisking vigorously after each addition to ensure there are no lumps. Once all the milk is incorporated, you'll have a smooth sauce. This is a classic French Béchamel sauce.

Thicken and Season: Bring the sauce to a gentle simmer and cook for 5-8 minutes, stirring often, until it has thickened enough to coat the back of a spoon. Stir in the Dijon mustard, grated nutmeg, salt, and pepper.

Add the Cheese: This is the most important step! Remove the saucepan from the heat completely. Let it cool for a minute. Add the grated Cheddar and Gruyère in handfuls, stirring well after each addition until the cheese is fully melted and the sauce is smooth and velvety. What works best for me is to wait until one handful has melted before adding the next.

Combine and Assemble: Pour the drained macaroni into the cheese sauce and stir until every piece of pasta is well-coated. Transfer the mixture to a large ovenproof baking dish (approx. 20x30cm).

Prepare the Topping: In a small bowl, combine the Panko breadcrumbs, grated Parmesan, and melted butter. Toss together with your fingertips until the breadcrumbs are evenly moistened.

Bake to Perfection: Sprinkle the breadcrumb mixture evenly over the top of the macaroni and cheese. Place the dish in the preheated oven and bake for 20-25 minutes, or until the sauce is bubbling and the topping is crisp and golden brown.

Rest Before Serving: Let the macaroni and cheese rest for 5-10 minutes before serving. This allows the sauce to set up slightly, making it even creamier.