Heat a nonstick skillet for 30 seconds over high heat. Sear the tenderloin for 1 minute then turn over and cook for an additional minute. Mix in the mushrooms and scallion. Cook for 2 minutes, stirring continuously. Set on a plate.
Pour the broth, wine, mustard, tarragon, parsley, and pepper into the skillet. Bring to a boil and cook for 3 to 4 minutes, stirring constantly. Add the beef mixture to the skillet and keep warm while preparing the potatoes.
Pour the diluted milk into a saucepan and bring to a boil. Add the potatoes, margarine, garlic powder, and pepper. Whip until it becomes a smooth consistency.
Spoon the mashed potatoes onto 2 plates and make a well in the center. Place the tenderloin in the well and pour some of the broth mixture over all. Sprinkle the chives on top.