In a 2-Quart Saucepan, melt the butter over medium heat. Add the flour and whisk until smooth, about 2 minutes. Gradually add the warm milk, whisking constantly to prevent any lumps from forming. Simmer over medium heat, whisking constantly, until the sauce is thick, smooth, and creamy, about 10 minutes (do not allow the béchamel sauce to boil). Remove from the heat and stir in ½ teaspoon of salt and a pinch each of white pepper and nutmeg. Season the sauce with more salt, pepper, and nutmeg to taste. (The sauce can be made up to 3 days ahead. Cool, then cover and refrigerate.)