Melt the butter in a heavy saucepan over a medium heat. Add the flour and cook until slightly golden, stirring occasionally. Pour in half the milk, stirring vigorously until smooth, then stir in the remaining milk and add the bay leaf. Season with a pinch of salt and plenty of pepper and nutmeg. Reduce the heat to med-low, cover and sinmmer gently for about 5 min. stirring occasionally.
Preheat the oven to 375 degrees. Generously butter a 2½ pint soufflé dish and sprinkle with parm cheese.
Remove the sauce from the heat and discard the bay leaf. Stir in both cheeses.
In a clean greasefree bowl, using an electric mixer or balloon whisk, beat the egg whites slowly until they become frothy. Add the cream of tartar, increase the speed and continue beating until they form soft peaks, then stiffer peaks that just flop over a little at the top.
Stir a spoonful of beaten egg whites into the cheese sauce to lighten it, then pour the cheese sauce over the remaining whites. Using a rubber spatula or large metal spoon, gently fold the sauce into the whites until the mixtures are just combined, cutting down through the center to the bottom, then along the side of the bowl and up to the top.
Gently pour the soufflé mixture into the prepared dish and bake for 25-30 minutes until puffed and golden brown. Serve at once.