Preheat oven to 200C/180C fan
Butter a gratin dish (20x20cm) and sprinkle over with half the parmesan
Melt butter in pan, add flour, stir briskly for 1 minute over moderate heat until it begins to go white
Add milk bit by bit, whisking briskly, then switch to wooden spoon to beat, bring the mixture to a bubble, reduce heat to medium/low and cook for about 8 minutes until thick and smooth
Add thyme, nutmeg, pepper and salt and cool the mixture, beating from time to time for about 5 minutes off the heat
Add beaten eggs and whisk well to combine, add cheese and mix to combine, add remaining parmesan to the top
Pour mixture into the prepared dish and bake in hot oven for around 35 minutes until puffed and golden
Remove from oven and serve immediately
