1. Place flank steak in large baking dish. Using fork, prick steak about 20 times on each side. Pour Teriyaki Marinade over steak, sprinkle with scallions. Turn steak over to coat both sides. Cover and refrigerate 1 hour or overnight.
2. Melt butter in large 12 inch skillet over medium heat. Add shallots and cook, stirring 1 minute. Add mushrooms and cook, stirring occasionally until mushroom liquid is evaporated, 15 minutes. Transfer to bowl. Add vinegar, soy and sugar to pan, bring to boil, reduce heat, return mushrooms to pan to heat through.
3. Preheat outdoor grill to medium high heat. Remove steak from marinade. Grill steak 6 to 8 minutes each side for medium rare. Let rest 10 minutes before slicing in 1/4 inch slices. Serve with mushrooms and bread.