Preheat the oven to 180°C/ 350°F/Gas mark 4. Grease a baking dish with oil.
Blanch the courgettes in boiling water for 2 minutes. Cool slightly.
Put the egg yolks, Parmesan and nutmeg in a bowl. Add the courgettes, and mix through.
Beat the egg whites until stiff. Fold into the courgette mixture.
Fill the baking dish with the mixture and bake for 25 minutes until the flan is golden on top. Remove from the oven, and serve piping hot.