To start, wipe the mushrooms clean and using a sharp knife, carefully slice them lengthwise, 1/4-inch thick slices. Transfer to a big plate.
In a small bowl, add the soy sauce/tamari, sake, sugar, onion and garlic powder. Mix until the sugar is dissolved, and the sauce is well combined. Pour the sauce over the mushrooms and using your clean hands, gently toss to get all mushroom slices nicely coated. Allow it to marinate for about 15 minutes
In a large non-stick skillet, heat 1 tablespoon of oil over med-high heat. Add about 1/3 of the marinated mushrooms and spread them in a single layer onto the heated skillet. Allow them to cook undisturbed for about 2-3 minutes. Flip and continue to cook for an additional 2-3 minutes, or until charred and tender. Set aside on a plate and repeat with the remaining mushrooms.
Once the last batch of mushrooms is done, return all mushrooms to the skillet, together with all their juices and remaining marinade from the bottom of the plate. Lower the heat and continue to cook, until most of the liquid is reduced and mushrooms are coated in a thick teriyaki sauce.
Serve hot over steamed rice and garnish with sesame seeds and green onions/scallions. Enjoy!