Break the spaghetti into lengths of about 5cm/2in and boil in salted water until al dente. Drain and toss in a few drops of olive oil to prevent sticking.
Melt the butter in a heavy saucepan and add the flour to make a roux. Cook for a minute or so,then add the hot milk, stirring until you have a thick, smooth bichamel. Allow to simmer gently for just a minute.
Remove from the heat, add the cheeses and mix until melted. Beat in the egg yolks. Stir in the cooked spaghetti until thoroughly incorporated. Season well with black pepper
Whisk the egg whites until stiff and fold gently and thoroughly into the mixture. Divide between four small souffli dishes (or two medium, or one large) and cook in a preheated oven at 170C/325F/Gas 3 for about 20-25 minutes until well risen and golden.