Preheat skillet to medium high.
Slice beef tenderloin medallions, and then lightly salt and pepper them on both sides. Preheat a medium-size skillet over medium heat. Add seasoned beef. Let cook 1 minute, then turn onto other side. Cook beef to desired temperature. Remove meat from skillet and set aside. Add butter, sliced mushrooms, and shallots. Sauté 2 minutes, then add garlic; deglaze the hot skillet with cognac, away from direct flames. Burn off any flaming alcohol. Place skillet back on burner, and then add the mustard, Worcestershire, heavy whipping cream, and demi-glace (or broth), bring to a boil, then reduce to a gentle simmer. Let reduce a few minutes, until slightly thickened. Just before serving, add the medallions back to the pan to reheat and serve. Sprinkle green onions and parsley on top.