Preheat the oven to 200 C, 180 Fan, Gas 6. Grease a 1.5 litre gratin dish with butter.
Sprinkle the grease gratin dish with half the parmesan and set aside.
Melt the butter in a saucepan over a moderate heat, then whisk in the flour and cook for 20 seconds until it bubbles a little. Whisk in the milk vigorously, then cook, whisking all the time, for 3-5 minutes until it comes to a boil.
Keep whisking over a low heat for two minutes until the mixture thickens, then remove from the heat and stir in the 1/2 tsp salt, the nutmeg and plenty of black pepper. Leave to cool for about ten minutes.
Now mix the beaten eggs into the white sauce along with the grated gruyere or cheddar and the herbs and mix well.
Pour the mixture into the buttered gratin dish and sprinkle with the remaining parmesan. Bake for 30-40 minutes until well puffed and nicely browned on top, Remove from the oven and serve immediately.