In a saute pan, melt the butter. Saute the onions for 2 minutes. Add the sliced mushrooms and continue sauteing for 2 to 3 minutes, or until the mushrooms start to wilt. Season with salt and pepper. Add the garlic and beef and continue sauteing for 2 minutes. Add the veal stock and bring up to a boil. Reduce to a simmer and cook for 3 to 4 minutes. Stir in the sour cream and parsley. Continue to cook for 3 minutes. Season with salt and pepper.
Yield: 4 to 6 servings