Steam the mushrooms until just tender, about 4 minutes. Place them in a sided dish, cool to room temperature, and then mix in the pesto, olive oil, vinegar, salt and pepper. Cover and marinate the mushrooms in the refrigerator 2 hours.
Divide the lettuce among 4 plates. Arrange the steak, tomatoes, cheese and mushrooms on top of the lettuce.
Drizzle the salads with the mushroom marinade mixture and serve.