White Lasagne

In a saucepan melt butter over moderately low heat. Stir in flour and cook roux, stirring, 3 minutes. Add milk in a stream and bring mixture to a boil, whisking until thick and smooth. Add salt and pepper to taste and simmer sauce over low heat, whisking occasionally, 10 to 12 minutes, or until thickened. Transfer sauce to a bowl and cover surface with a buttered round of wax paper.