Tear the romaine lettuce into bite size pieces and add to a large bowl; toss with balsamic vinaigrette. Set aside.
Heat the olive oil in a non-stick skillet over medium heat; add onions and cook until tender and caramelized.
Meanwhile, slice the steaks against the grain in thin slices.
Once onions are complete, add onions and grape tomatoes to romaine lettuce. Toss salad and divide between two plates. Season salad with pepper, to taste. Arrange steak slices atop salad, sprinkle with cheese and serve.
Mix the vinegar, lemon juice, salt and pepper (to taste). Gradually whisk in oil.