1 1/2 tsp. vegetable oil
1 large onion, chopped
1/2 tsp. dried tarragon
1/4 tsp. nutmeg, freshly grated
1 lb. mushrooms, trimmed and sliced
1/2 cup all purpose flour
3 1/2 cups nonfat beef broth
1 cup reduced fat sour cream
1 cup 1% skim milk
salt and fresh ground black pepper to taste
pinch of cayenne or dash of Tabasco sauce
In a large heavy saucepan, heat oil over medium low heat. Add onions and saute until soft and translucent, 5 to 7 minutes. Add tarragon and nutmeg and cook for 1 minute more. Stir in mushrooms, cover pot and let vegetable mixture steam for about 5 minutes, until mushrooms exude their moisture.
Sprinkle flour over the vegetable mixture. Increase heat to medium and cook, stirring, for 3 or 4 minutes. Gradually stir in beef broth, stirring and scraping any flour that may have clung to the pot. Simmer, stirring occasionally, until thickened and smooth, 5 to 7 minutes.
Stir together sour cream and milk until smooth, whisk into the mushroom mixture and return to a simmer. Season with salt and pepper and cayenne or Tabasco, if desired, and serve.