1. Preheat the oven to 200°C/400°F/gas mark 6. Bring a large pan of salted water to the boil, add the pasta and cook for 8-10 minutes or until al dente. Drain and set aside.
2. Whilst the pasta is cooking, heat the milk and nutmeg in a small saucepan until simmering then reduce the heat and keep warm. In a separate pan, melt the butter and whisk in the flour. Cook for 2-3 minutes until the mixture starts to turn a light brown colour. Gradually whisk in the milk; don’t add too much at once or the sauce will turn lumpy. Once all the milk has been incorporated bring the sauce to a simmer and cook gently for 2 minutes, whisking constantly. Remove the pan from the heat and whisk in the mustard (if using) and half of the cheese.
3. Add the cooked pasta to the sauce and stir until evenly coated. Pour the mixture into a 2.6 litre rectangular Pyrex dish then sprinkle the remaining cheese over the top.
4. Bake in the oven for 20-25 minutes or until the cheese is golden brown and bubbling. Remove from the oven and leave to cool for a few minutes before serving.