Sauté onion in oil until transparent. Add beef and cook until browned. Drain excess fat.
Blend in soup, catsup or chili sauce, and sliced mushrooms, if desired. Heat through. Remove from heat.
Add sour cream (at room temperature) all at once; blend well. Heat gently to serving temperature. Add salt to taste. Serve over hot noodles or rice. Makes 4 servings.