Directions

1. In a small bowl, whisk together vinegar, brown sugar, Worcestershire sauce, soy sauce, oil, garlic, and crushed red pepper flakes. Set aside 1/4 cup marinade in the refrigerator.
2. Stem and seed bell peppers and cut into quarters. Place red peppers, steak, and mushrooms in a shallow dish or large zip-top bag. Pour remaining marinade over the vegetables and meat. Cover dish or seal bag and refrigerate at least 4 hours and up to overnight, turning occasionally.
3. Preheat a grill to medium-high heat (400° to 450°F). Discard balsamic marinade from steak and vegetables. Sprinkle steak, peppers, and mushrooms with salt and pepper. Shuck corn and trim root ends from green onions.
4. Grill steak, corn, peppers, mushrooms, and green onions in batches with the lid closed: Grill steak, turning it once, until medium-rare (130° to 140°F, or done the way you like), 12-14 minutes total. Transfer to a cutting board, tent with foil, and let rest about 15 minutes. Grill corn, basting with the reserved marinade, 8-10 minutes, turning occasionally; transfer to board. Grill mushrooms and peppers, turning once, until tender and browned, 4-8 minutes total; transfer to board. Grill green onions until charred and tender, about 1 minute, then set on board.
5. Slice steak across the grain, slice mushrooms about 1 inch thick, and serve steak with vegetables.