Preheat the oven to 200C/400F/Gas 6.
Melt the butter in a saucepan over a medium heat. Stir in the flour and cook for 30 seconds. Remove from the heat and gradually whisk in the milk.
Bring to the boil and stir in the spinach and most of the cheese.
Allow to cool slightly. Beat in the egg yolks and season well.
In a large bowl whisk the egg whites to medium peak.
Mix two large spoonfuls of egg to the milk mixture to slacken. Fold in the remaining egg whites and sprinkle with the Edam.
Transfer to an oven-proof frying pan. Place in the oven and cook for 10 minutes.
Serve the soufflé in the frying pan.