Hi Carol, if the white sauce in your creamed onions is a type of béchamel (kind of like this https://food52.com/recipes/2759-bechamel-sauce), yes, you can absolutely make it in advance—I'd suggest up to a day ahead of when you want to use it. Just make sure you cover it well with foil or plastic wrap before putting it in the fridge for the night, placing the foil or wrap directly against the surface of the sauce, so a "skin" doesn't form at the top as it refrigerates. Then, just reheat the sauce over gentle heat before you use it in your creamed onions, whisking vigorously and adding a bit more milk if it's slightly stiffer than you like.
You have been helpful beyond words--- I am back to the kitchen to make the sauce now instead of in the AM. Marvelous-- now I have a little more time to get to my dinner destination tomorrow. A super Happy Thanksgiving to you and yours!!!
Please be sure to use plastic pressed directly onto the surface of your sauce. The plastic will prevent a skin from forming. Don't use foil. If pressed to the surface, lactic acid in the milk will react with the foil, causing a grey color to result.
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Hope this helps! Happy Thanksgiving!