In a small saucepan, melt butter; add the onions and saute until the onions and butter are a rich brown in color. Add the truffle flour and the all-purpose flout and saute another couple of minutes.
Whisk in the milk and continue whisking until sauce has lightly thickened. Cover and let simmer for an additional 10 minutes.
Remove from heat and salt to taste.
Makes approximately 1 cup Truffle Onion Sauce.
* If you can not find truffle powder, all-purpose flour and a few drops of white truffle oil can be substituted.
Truffle Onion Sauce Recipe: https://whatscookingamerica.net/sauces_condiments/truffleonionsauce.htm