Position the racks to the lower and upper thirds of the oven.
Preheat the oven to 425 F.
In a medium bowl make your sauce. Whisk together the tamari sauce, sesame oil, balsamic vinegar, garlic, ginger, arrowroot starch, honey. Set aside.
On the sheet pan, arrange your mushrooms, broccoli, bell peppers and onion. Drizzle with a little avocado or coconut oil. It's ok if your sheet pan is full. The vegetables will get smaller as they roast.
Roast on the lower rack for 10 minutes.
Remove from the oven. Place the chicken over the vegetables in a single layer. Drizzle your marinade over everything.
Return the sheet pan to the oven and roast on the lower rack for another 10 minutes.
Turn on the broiler and move the sheet pan to the upper rack. Broil for 2-5 minutes to brown the chicken and veggies a little bit.
Remove sheet pan from the oven. Stir the veggies and chicken to mix everything up. Top with sliced scallions.
Serve on own or with cauliflower rice or white rice.