Heat the oven to 375°F (190°C). Lightly butter a 2.5-liter baking dish. Bring a large pot of salted water to a boil and cook the macaroni about 2 minutes less than the package’s al dente time. Drain well.

Meanwhile, melt the 45 g butter in a saucepan over medium-low heat. Stir in the flour and cook, stirring, for 1 minute without browning. Add the milk gradually while whisking to keep the mixture smooth.

Bring the sauce to a gentle simmer, whisking, and cook for 3–5 minutes until it lightly coats a spoon. Turn off the heat. Stir in the cheddar and Gruyère a handful at a time, then add the mustard, salt and pepper. Taste and adjust the seasoning.

Fold the drained macaroni into the sauce. Transfer to the prepared dish; the mixture should look generously sauced rather than dry.

Mix the panko with the 15 g melted butter and scatter evenly over the pasta. Bake for 20–25 minutes, until the edges bubble and the crumbs are golden. Avoid baking longer just to make every spot equally dark.

Let stand for about 5 minutes, then add the optional parsley and serve. Refrigerate leftovers in shallow containers within 2 hours, or within 1 hour above 90°F (32°C). Use within 3–4 days and reheat to 165°F (74°C).