Preheat the oven to 200 degrees C. Grease and line the base of a 23×33cm Swiss roll tin with baking paper. Put the spinach in a sieve and press to drain the liquid, then scoop into a bowl. Add the egg yolks and nutmeg, season and mix well.
Whisk the egg whites until stiff, then fold into the spinach mixture with a metal spoon. Spoon into the lined tin and spread evenly. Bake for 15 minutes until firm to the touch.
Meanwhile, melt the butter in a large saucepan. Add the mushrooms and fry gently for 5 minutes until tender. Add the flour and cook over a low heat, stirring with a wooden spoon, for 1 minute. Remove the pan from the heat and gradually blend in the milk, stirring after each addition. Return to the heat and bring to the boil, stirring all the time, until the sauce thickens and is smooth. Remove from the heat and stir in the cheese, mustard and salt and pepper.
Remove the baked spinach mixture from the oven and, working quickly, spread the mushroom sauce evenly over the surface. Roll up from one of the short ends by gently lifting the baking paper. Serve hot, cut into slices.