Pick over and destalk the spinach, then blanch in rapidly boiling salted water for 60 seconds. Refresh in cold water, drain and squeeze out all the moisture you can.
Make a béchamel sauce: melt the butter in a heavy pan and stir in the flour. Cook gently, stirring, for about 1 minute, then add the milk and cook, stirring, until thick and smooth. Cook gently for 20 minutes, stirring regu