Wash spinach in several changes of cold water until there is no trace of sand on them. remove stem from leaves. Drain well. Chill in the refrigerator. Wash the mushrooms and dry them well. Trim off the bottoms of the stems. Cut the mushroom into thin slices. Place the spinach into a medium bowl. Toss the mushroom and spinach together. Portion the salad onto chilled salad plates or bowls. Garnish with red onion and tomatoes.
For the dressing: Add all ingredients into small bowl. Whisk ingredients together using a wire whip for about one minute. Season with salt and pepper, then spoon over salad.