Sauté onion in melted butter until transparent. Remove and discard onion.
Mix flour with butter with a fork over low heat until a smooth mix is made. Remove from heat and let cool to just warm.
Add warm milk or even out of the refrig. Return to medium-low heat and stir with fork to mix. Then whisk smooth, and continue to watch and whisk as needed until flour is cooked (ca. 15 minutes).
Thyme or nutmeg are sometimes added with the alt and white pepper.
Yields: