Put the butter, water and milk into a saucepan and bring to a gentle simmer. Take off the heat and add the flour, stirring continuously, until you have a smooth ball of dough that pulls away from the side of the pan. Cook over a low heat for 5 minutes, stirring constantly, then take it off the heat and leave to cool slightly. After another 5 minutes or so, add the eggs one at a time, beating vigorously after each addition. Once the mixture is well combined, glossy and smooth, spoon the dough into a piping bag fitted with a large round tip (1.5 centimetres is ideal).
Bring a large pan of salted water to the boil, and line a large tray. Cook the gnocchi in batches, using the piping bag and a butter knife to snip 3 centimetre lengths of dough into the boiling water. Let the gnocchi rise to the surface then poach for 3 minutes before removing them with a slotted spoon. Leave the gnocchi to cool in a colander while you make the Creamy Havarti sauce.
Preheat the oven to 350°F. Melt the butter in a saucepan then add the flour, stir well, and cook for a few minutes before gradually adding the milk, mixing well after each addition to ensure a smooth sauce. Cook the sauce for 5 minutes on a low heat, stirring often, then grate in the nutmeg and add the Creamy Havarti. Stir until the cheese has melted and then take it off the heat.
Arrange the cooled gnocchi in an oven dish and pour the Creamy Havarti sauce on top. Dot with extra cubes of cheese and then bake in the hot oven for 20 minutes until the sauce is bubbling and the cheese has melted. Serve immediately.