Melt butter, if using, in very hot skillet. Brown meat in skillet with mushrooms and onion. If using ground beef or turkey beef sure to stir with a fork to break up meat. Add thyme, salt and pepper.
3 tablespoons butter, 1 pound well trimmed bite-sized strips tender beef, 3 cups fresh mushrooms, 3 green onions, ¼ teaspoon powdered thyme, salt and pepper
Reduce heat to medium. Add sherry and cook down until liquid is reduced to about ¼ cup. Add broth and heat to just boiling.
¾ cups dry cocktail sherry, ¾ cups beef broth
Mix cornstarch with water (stir with a fork until there are no lumps) and pour all at once into mixture in skillet. Stir constantly until liquid has thickened.
1 tablespoon cornstarch, 3 tablespoons cold water
Stir in sour cream or yogurt just before serving. Serve over rice or egg noodles.
1 cup sour cream or plain yogurt
Notes
To make without alcohol: use a total of 1 ½ cups of beef broth plus one tablespoon of apple cider vinegar.