1. Peel and slice the onion. Cut the meat into bite-sized strips.
2. Place onion, oil and mushrooms in bowl fitted with mixer. Launch the P1 simmer program at 270°F for 5 minutes. Dredge the meat in flour, salt and pepper mixture.
3. At the end of the program, add meat, tomato paste, wine and mustard. Launch the P3 simmer program at 210°F for 10 minutes.
4. Add the sour cream. Launch the P3 simmer program at 210°F for 5 minutes. Garnish with fresh parsley, if desired.