Cook sirloin in 4 tablespoons butter. Saute onions 10 minutes in 2 tablespoons butter. Saute mushrooms 5 minutes in remaining 2 tablespoons butter. Mix all together and add tomato paste, sour cream and seasoning. Cook thoroughly. Cover, cook over low heat 30 minutes, add marsala wine. Simmer 5 minutes longer. Serve over noodles or wild rice. Serves 6 or more.