Slice the chicken breast into thin, bite-sized strips and season evenly with the sea salt and black pepper.
Clean the cremini mushrooms and slice them thinly; chop the green onions, keeping the white bases and green tops separate.
Heat the avocado oil in a large skillet or wok over medium-high heat until it begins to shimmer.
Add the chicken strips to the skillet in a single layer and sear for 3-4 minutes until golden brown and fully cooked through.
Remove the chicken from the pan and set aside on a plate; add the mushrooms to the same skillet and sauté until they release their moisture and turn deep brown.
Stir in the minced garlic, grated ginger, and the white parts of the green onions, cooking for 60 seconds until fragrant.
Return the cooked chicken to the pan and pour in the tamari and toasted sesame oil, tossing all ingredients to coat in the glaze.
Serve the stir-fry immediately over the warm brown rice and garnish with the remaining green onion tops.