Cook the quinoa according to the instructions on the package. Use vegetable broth instead of salted water.
Meanwhile heat the olive oil in a medium pan and sauté two of the three green onions together with the mushrooms for about three minutes. Set the remaining green onion aside.
For the dressing combine all ingredients in a salad bowl and mix until smooth.
Stir in the mushrooms, the green onions, the tomatoes, the cooked quinoa, and the spinach. Sprinkle with the remaining green onion.