Tenderize the beef schnitzel by beating them, then trim the edges. Season both sides with salt and pepper, then brush with tarragon mustard. Dip one side in flour.
Heat oil in a large pan over medium-high heat. Fry the schnitzel floured-side down for 3–4 minutes until golden, then flip and fry for 2–3 minutes. Remove to a plate.
In the same pan, sauté the diced onion and soup greens over medium heat for 5 minutes. Deglaze the pan with red wine and simmer for 5 minutes, then add the beef broth. Return the schnitzel to the pan.
Reduce heat to low and simmer uncovered for approximately 2 hours until the schnitzel is very tender.
Remove the schnitzel to a serving plate. Using an immersion blender, puree the sauce in the pan until smooth. Over low heat, stir in the sour cream (do not boil or it will curdle), then season with salt and pepper to taste.
Return the schnitzel to the sauce, warm gently without boiling, and serve with buttered ribbon noodles and green salad.