Preheat grill to medium high. Rub steak with olive oil on each side. Season with black pepper. Grill filet 5 minutes per side for medium rare. While steak is grilling, cut mushroom stem even with cap. Discard stem. Slice mushroom cap and green onions.
In a large skillet, melt margarine over medium. Add mushroom slices and green onions and cook for 5 minutes or until vegetables are tender. Stir in broth and port. Bring to boil. Remove from heat. Thinly slice steak diagonally and serve with sauce.