Preheat your oven to 180°C (160°C fan). Cook the macaroni in a large pot of well-salted boiling water according to package directions, but undercook it by about 2 minutes. It will finish cooking in the oven. Drain well and set aside.

While the pasta is cooking, make the sauce. In a large saucepan or Dutch oven, melt the butter over a medium-low heat. Once melted, whisk in the plain flour to form a thick paste, known as a roux. Cook this, stirring constantly, for 2 minutes. This step is vital to cook out the raw flour taste.

Gradually add the warm milk to the roux, a little at a time, whisking vigorously after each addition to ensure a smooth, lump-free sauce. What works best for me is to add the first 200ml or so very slowly until it's fully incorporated, then you can add the rest a bit more quickly.

Once all the milk is incorporated, bring the sauce to a gentle simmer and cook for 5-7 minutes, stirring often, until it has thickened enough to coat the back of a spoon.

Remove the saucepan from the heat. This is a crucial step to prevent the cheese from splitting and becoming grainy. Add the mustard powder, cayenne pepper, and nutmeg. Season generously with salt and black pepper.

Add the grated Cheddar and Gruyère to the sauce (reserving a small handful for the topping if you like extra cheese) and stir until the cheese is completely melted and the sauce is smooth and glossy.

Tip the drained pasta into the cheese sauce and stir well until every piece is thoroughly coated.

Pour the mixture into a large ovenproof baking dish and spread it out evenly.

In a small bowl, mix the breadcrumbs, grated Parmesan, and olive oil. Sprinkle this mixture evenly over the top of the macaroni cheese.

Bake for 20-25 minutes, or until the top is golden brown and the sauce is bubbling deliciously around the edges. Let it stand for 5 minutes before serving.