Bring 1 cup water to a boil; add mushrooms, cover, and boil 2 minutes.
Drain and place in glass bowl.
Whisk together remaining ingredients except spinach in small bowl and blend well.
Pour over hot mushrooms and chill at least 1 hour.
Tear spinach into bite-sized pieces and place in salad bowl.
Add mushrooms and enough marinade to dress, but not drown, greens.
Toss lightly and serve.