First, make the white sauce. Melt the butter in a frying pan, then sprinkle over the flour. Mix for 30 seconds, then gradually add 300ml of the milk, whisking as you go. Season with salt, pepper and add a pinch of nutmeg. Bring slowly to the boil and leave to thicken a little on a gentle heat for 2 mins. Cook the pasta according to the packet instructions. Drain and place it back in the saucepan, off the heat. Add the reamining 100ml of milk, and 100ml of the white sauce. Mix well.