Reduce the heat to medium and add the remaining 1 teaspoon oil. Add onion and sauté until softened and lightly colored, about 3 minutes. Reduce heat to low, add mushrooms and sauté until they are just beginning to give off their moisture, 2 to 3 minutes more. Add flour and cook for 1 minute, stirring. Stir in the wine and let evaporate, about 30 seconds. Stir in broth, bring to a simmer, stirring, and cook until thickened, about 2 minutes. Stir in mustard, sour cream and parsley. Season to taste with salt and pepper. Add the reserved beef and cook just until heated through. Serve over the cooked noodles.