Stir vinegar with soy sauce, Worcestershire sauce and honey. Add flank steak and marinade to a resalable bag. Marinate, refrigerated, for at least 1 hour, and up to 24 hours.
Preheat George Foreman Grill for 5 minutes with lid closed. Remove steak from marinade and reserve. Season steak with 1/2 tsp salt and 1/4 tsp pepper.
Place steak on grill, cook for 6 to 8 minutes or until internal temperature reaches 145°F for medium rare. Tent with foil and let rest for 10 minutes before slicing. Transfer marinade to a small saucepan set over medium heat; bring to a boil for 1 minute and cook until thick and syrupy. Meanwhile, toss mushrooms with thyme, garlic and the remaining salt and pepper. Place on grill and cook for 3 to 5 minutes or until grill marked and tender.
Thinly slice steak across the grain. Drizzle with cooked marinade. Top with mushrooms.
Tip: Serve on a bed of lightly dressed arugula, if desired.
For the evolve grill, preheat grill to 375° and cook for 5-7 minutes depending on your desired doneness.
For the Indoor|Outdoor grill, preheat the grill to Setting 3 and grill covered for 7 minutes.