In a saucepan, melt butter, whisk in flour. Cook for about 2 minutes then turn off heat & slowly begin to add milk. Continue whisking until all milk has been added then turn on heat & bring sauce to a boil. Whisk constantly until thickened then add 3/4 of the Gruyere cheese & stir until cheese is melted. Spread the bechamel sauce evenly in a 9 x 13-inch baking dish.