In a 12-inch nonstick skillet, combine onions, garlic, wine, and butter. Cook over medium heat, stirring frequently, 6 to 8 minutes or until onions are tender. If liquid begins to evaporate, add a bit of water.

In a cup, combine cornstarch with 1/4 cup milk; stir to mix well. Stir remaining 2-1/2 cups milk into skillet. Bring liquid in pan to a boil over medium-high heat. Stir in cornstarch mixture, stirring until mixture thickens. Boil 1 minute, stirring. Reduce heat so that sauce does not boil.

Stir in Parmesan cheeses. Stir in sour cream, mixing well and whisking if necessary to smooth. Stir in 1/4 cup chives, Worcestershire sauce, mustard, and pepper, and stir well. Add salt, if using. Cook over very low heat an additional 2 to 3 minutes until flavors are well blended.