1. Heat oil in heavy skillet over medium heat; cook mushrooms, garlic and onion, stirring occasionally, until onion softens and water from mushrooms cooks off. Stir in balsamic vinegar; cook, stirring for 3 to 5 minutes or until vinegar is absorbed. Set aside.
2. Rub steak with pepper to your liking. Grill or pan-fry over medium-high heat on lightly oiled grill or pan for 3 to 4 minutes per side.
3. Slice steak thinly on the diagonal. Drape slices of steak on each baguette bottom piece. Top each with mushroom mixture, goat cheese and arugula. Makes 4 open sandwiches.