In a large sauté pan, warm the sesame oil or extra-virgin olive oil over medium heat. Add the mushrooms and sauté until lightly golden, 3 to 4 minutes.
Add the tamari and sauté for 2 minutes more. Deglaze the pan with the mirin to release the bits from the bottom of the pan. Turn off heat.
In a large bowl, whisk together the ingredients for the dressing. Using a spatula, stir in the grains and fully coat. Add the mushrooms and 3 cups of the greens, and gently fold to coat well with the dressing. Top with the avocado, reserved greens, and gomashio.
