In large pot, cook sliced mushrooms with the 4 tablespoons of butter over medium heat. Drain the water that cooks out of the mushrooms until they give no more (save the mushroom stock for the soup).
Sauté the chopped onions in small amount of oil until browned.
In heavy skillet, melt 1/2 cup butter over low heat. Add the sifted flour, stirring constantly until thick.
Add the sour cream and stir until the mixture is warm. You may add more flour if necessary to thicken. Combine all the ingredients and stir well.