Grill or broil flank steak; allow to cool, then cut into 2" julienne strips. Heat 3 tablespoons oil in skillet, add lemon juice and saute mushrooms in mixture. Set aside to cool. In a bowl, whisk together remaining olive oil, vinegar, garlic, herbs, mustard, salt and pepper. Add steak, mushrooms and sliced onion. Toss and let mixture marinate for an hour. Arrange a bed of lettuce on a large platter and top with steak-mushroom-onion-vinaigrette. Sprinkle with parsley. Garnish with cherry tomatoes.