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.