Heat the olive oil in a large skillet over medium heat. Add in the diced onion and sauté for about 3-4 minutes, until the onions become translucent.
Add in the mushrooms, salt, garlic, green onions and ginger and sauté for an additional 5-6 minutes until the mushrooms cook down.
Once the mushroom mixture is ready, add the mixture to a food processor with the walnuts, brown rice, oats, Kikkoman® Teriyaki Marinade & Sauce, black pepper and red pepper flakes. Blend together for about 30-40 seconds. Scrape down the sides and pulse again until a sticky mixture forms.
To create the meatballs, spoon about 2 tbsp worth of the mixture and roll it into balls with your hands. I like to wet my hands for this process if the batter is too wet to work with.
Layer the meatballs on a parchment lined baking sheet and spray them with cooking spray. Bake for 25 minutes, flip and bake for an additional 15 minutes.
Add the teriyaki sauce ingredients to a small bowl and whisk together. Pour it into a saute pan and heat it on medium heat, letting it cook for about 2-3 minutes until it starts to thicken. Once thick, add in your cooked mushroom meatballs and toss them gently to coat.