6 tablespoons butter
1/4 cup minced onion
2 pounds beef tenderloin, cut into thin 1- by 1 1/2-inch pieces
1/2 pound mushrooms, wiped clean and thinly sliced
Salt and pepper to taste
1 1/2 cups sour cream at room temperature
Melt 3 tablespoons of the butter in a heavy skillet. Add the onion and cook slowly until transparent; remove and set aside. Turn the heat to medium-high, add the beef and cook briefly, turning to brown on all sides.
Remove the beef and set aside with the onions. Add the remaining 3 tablespoons of butter to the skillet. Stir in the mushrooms, cover and cook 3 minutes. Season with salt and pepper. Whisk the sour cream and add to the pan, but do not allow it to boil. Add the beef and onion to the pan and heat through. Serve with brown rice. - Serves 6