¼ cup unsalted butter
¼ cup all-purpose flour
2 cups milk
1 small onion studded with 2 Or 3 cloves, optional
1 small bay leaf
dash dried leaf thyme, crumbled
salt and white pepper to taste
nutmeg, to taste
In a medium heavy saucepan, melt butter over low heat. When butter starts to foam, add the flour all at once, mixing well with a wooden spoon. Cook over low heat 3 to 4 minutes, stirring constantly to incorporate and cook flour. Remove pan from heat and let stand, up to 15 minutes.
Meanwhile, in a medium saucepan, scald milk (heating it until just below boiling point).
Return saucepan with roux to medium-low heat. Add all of the scalded milk at once (to avoid the formation of lumps). Simmer, stirring gently with a wire whisk or wooden spoon. Add studded onion, bay leaf and thyme sprig.
Cook, stirring, over low heat, 15 to 20 minutes, until smooth and thickened. Strain sauce through fine-mesh strainer. Add salt, white pepper and nutmeg to taste.