To make sauce, in 2-quart heavy saucepan, melt butter over medium heat. Using whisk, stir in flour until blended. Add milk; cook and stir until mixture comes to a boil. Stir in nutmeg and salt. Reduce heat to low; simmer about 15 minutes, stirring frequently, until mixture thickens enough to coat back of wooden spoon. Check seasoning level; add additional salt and nutmeg if necessary.