Preheat the oven to 180°C / gas mark 4.

Cook the pasta according to the packet instructions. Drain and set aside.

Meanwhile, melt the butter in a large pot (one that’s big enough to take all the pasta later on), then add the flour. Whisk it together to form a paste and cook this roux for 2–3 mins over a medium heat, until golden brown. Add in a little milk and whisk to combine, then gradually pour in the rest of the milk, whisking constantly.

Simmer until the white sauce is smooth and has thickened slightly, which should take about 5 mins, stirring regularly to make sure it doesn’t burn. Stir in the thyme, mustard and a generous grinding of black pepper, then add the cheeses. Stir until the cheese has all melted and the sauce is smooth again.

Remove from the heat and add in the drained, cooked pasta, stirring until all the pasta is coated with the sauce. Transfer the pasta to one large baking dish, individual gratin dishes or large ramekins.

To make the herb breadcrumb topping, melt the butter in a frying pan over a medium heat. Add the breadcrumbs and stir until coated with the butter, then add the thyme and season with salt and pepper. Fry for about 5 mins, stirring occasionally, until the breadcrumbs are golden brown. Remove from the heat.

Place the dish(es) on a baking tray and sprinkle the breadcrumbs on top. Bake in the oven for 15–20 mins, until the pasta is piping hot and the breadcrumbs are crispy.

Serve straightaway. This is best eaten on the day it’s made.