1.Heat oven to warm setting. Sprinkle meat with salt and pepper.
2.In a large nonstick skillet heat oil over medium-high heat. Brown meat in batches, about 1 minute per side. Transfer to a platter and place in oven.
3.Melt butter in skillet; add onion and cook 3 minutes, until softened. Add mushrooms. Cook 10 minutes, stirring occasionally, until mushroom liquid evaporates.
4.Add wine; cook 5 minutes. Stir in beef broth and cook 10 minutes, until mushrooms are coated with a thick sauce. Stir in sour cream and mustard. Add meat and accumulated juices.
5.Reduce heat to low and cook 2-3 minutes, until meat is heated through. Season to taste with salt and pepper.