Prepare soup according to instructions on box, except reduce water by 1 cup. (use 3 cups water instead of 4) Add diced onion and simmer about 20 minutes.
Stir in (raw) stew meat. Turn mixture into large casserole dish (uncovered) and bake at 250 degrees for 4 hours. (check if it’s getting brown; stir as necessary.
30 minutes before serving, add optional mushrooms if desired
10 minutes before serving, stir in sour cream and let it heat through
Serve over noodles, wild rice, or mashed potatoes.