SPINACH: Cook spinach briefly by either boiling or steaming.
Drain and squeeze out excess moisture.
Mix with 1 cup white sauce and warm over low heat, 3-to-5 minutes.
WHITE SAUCE (BECHAMEL): Melt the butter in a saucepan over low heat.
Add the flour and cook, stirring with a wooden spoon, 2-to-3 minutes.
Remove from heat, add the hot milk gradually, stirring constantly with a whisk.
Return to the heat and simmer, stirring constantly, until thickened, 2-to-3 minutes.