Preheat a greased barbecue plate on high heat. Place mushrooms in a large bowl and spray generously with oil, tossing to coat.
Barbecue mushrooms, turning occasionally, for 3—4 minutes until golden. Transfer to a large bowl.
To make sweet soy dressing, combine all the ingredients in a screw-top jar. Shake to combine. Pour the dressing over the warm mushrooms and gently toss to combine. Set aside for 20 minutes or until cooled.
Add spinach, green onions and sesame seeds to mushrooms. Season with pepper. Gently toss to combine. Transfer to a serving bowl and serve immediately.