Baked Macaroni Cheese with a Crunchy Crumb

Heat the Oven, Butter the Dish. Set the oven to 190°C (170°C fan) at least 20 minutes ahead so it reaches full heat before baking, and rub the 10g butter around the baking dish. A properly preheated oven lets the crumb colour and the sauce bubble at the edges without the centre lagging behind.

Cook the Macaroni. Bring 3 litres of water to the boil with the 15g salt, a light 0.5% since the cheese itself is salty, and cook the macaroni for 2 minutes less than the packet time, stirring well in the first minute so it doesn't stick together. Drain without rinsing, since the starchy coating left on the pasta helps the sauce cling. Cooking it slightly underdone means it keeps some bite as it finishes softening in the oven.

Build the Roux. Melt the 40g butter in the saucepan over medium heat, whisk in the 30g flour and cook for 1 minute without letting it colour. Pour in the 700ml milk in a thin stream while whisking constantly, then simmer, whisking into the corners of the pan, for 3-4 minutes until it lightly coats the back of a spoon. Cooking the flour first drives off its raw taste, and whisking the milk in gradually stops lumps forming as the starch swells and thickens the liquid.

Melt in the Cheese. Stir in the mustard powder, then drop the heat to its lowest setting and whisk in the Cheddar and Gruyère a handful at a time until smooth. Keeping the heat low matters here, as cheese added to a boiling sauce can turn grainy and oily, while the flour already in the sauce helps hold the melted fat in a smooth emulsion. Season with the salt and pepper.

Stir Pasta into Sauce. Tip the macaroni into the sauce and stir to coat; it will look quite loose and wet, but that's fine. The pasta will soak up some of the liquid as it bakes, so a thinner sauce now means a creamier finished bake rather than a dry one.

Transfer and Level. Tip the sauced macaroni into the prepared dish and spread it into an even layer with a spatula. An even surface means the crumb topping toasts at the same rate all over instead of burning in thin spots.

Scatter the Crumb. Toss the panko breadcrumbs with the melted 30g butter until evenly coated, then scatter over the macaroni in a thin, even layer. Coating the crumbs in butter first helps them crisp and brown in the oven rather than drying out and staying pale.

Bake Until Golden. Bake for 25-30 minutes until the sauce is bubbling at the edges of the dish and the crumb is a deep golden brown all over. The bubbling edges show the sauce has heated through fully, while the colour on top confirms the crumb has crisped.

Rest Before Serving. Let the dish stand for 10 minutes out of the oven before serving. Resting allows the sauce, still hot from a rolling bubble, to settle and thicken slightly so it clings to the pasta rather than pooling when spooned out.