In a large pot, cook sliced mushrooms with 1/2 stick of butter over medium heat. Drain the water that cooks out of the mushrooms until they give no more. (reserve this stock for soup.)
Saute chopped onions in small amount of oil until brown. In a cast-iron skillet or wok, melt 1 stick of butter over low heat. Add sifted flour until thick, stirring constantly. Add sour cream and stir until mixture is warm. Add more flour if necessary to thicken. Combine all ingredients and stir well. Salt and pepper to taste. Serve over fresh mashed potatoes.