Allowing the steak to stand before cutting it preserves the juices and makes for easier slicing. To enhance the steak's tenderness, cut the meat diagonally across the grain.
1/2 cup balsamic vinegar
1/3 cup olive oil
1 tablespoon Worcestershire sauce
1/2 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
2 garlic cloves, minced
1 (1 1/2-pound) flank steak, trimmed
6 large portobello mushroom caps
1 large red onion, cut into 1/2-inch-thick slices
6 cups mixed salad greens
1 cup halved cherry tomatoes
How to Make It
Combine first 6 ingredients in a small bowl, stirring with a whisk until blended.
. Combine steak, mushrooms, and onion in a large zip-top plastic bag. Add 1/2 cup vinegar mixture to bag; seal. Marinate in refrigerator 2 1/2 hours, turning occasionally. Reserve remaining 6 tablespoons vinegar mixture.
. Prepare grill.
. Remove steak and onion from bag, and place on grill rack coated with cooking spray. Grill 6 to 8 minutes on each side or until steak is desired degree of doneness and onion is tender. Remove mushrooms from bag; discard marinade. Place mushrooms on grill rack; grill 4 to 5 minutes on each side or until tender. Cut mushrooms into 1/2-inch-thick slices. Let steak stand 5 minutes. Cut steak diagonally across grain into thin slices.
. Chop onion; combine onion and mushrooms in a medium bowl. Combine mixed greens and tomatoes, and arrange evenly on plates. Top evenly with mushroom mixture. Arrange steak evenly over mushroom mixture; drizzle each serving evenly with reserved vinegar mixture. Serve immediately.