Prep Time: 20
Cook Time: None
Serves: Serves: 4
Combine mustard, vinegar and tarragon in small bowl. Gradually add oil, whisking until slightly thickened. Combine mushrooms, onion and tomatoes in medium bowl. Toss with dressing, let stand 15 minutes.
Arrange spinach leaves on 4 individual serving plates. Divide marinated vegetables among plates. Top each salad with 1/4 of the cheese. Sprinkle with pepper, if desired.