Soak the mushrooms - Pour the mushrooms in a bowl and cover them with cold water. Let them soak for at least two hours.
WHEN READY TO COOK
Prep the chicken - Cut the chicken into bite-sized pieces. Toss with cornstarch, salt, and pepper. Set aside.
Prep veg - Wash and trim the celery, and chop it into 1/2-inch pieces. Wash and trim the scallions, then cut them into 2-inch pieces, separating the white and light green pieces from the scallion greens. Set aside.
Make the sauce and prep the mushrooms - Whisk all of the sauce ingredients together in a small bowl. Drain the mushrooms and give them a rough chop.
Stir-fry, in stages - Heat the grapeseed oil in a large wok over medium-high heat. Add the chicken and sauté until it's cooked through and beginning to brown on the edges. Scoop the chicken out of the wok and set aside on a plate. Add the celery and the root ends of the scallions to the wok and sauté until crisp tender. Add the mushrooms and scallion greens to the wok and cook a few minutes more.
Finish - Return the chicken to the pan with its juices, stir in the sauce, and cook until everything is evenly coated and the sauce begins to thicken. Off the heat, drizzle with sesame oil and serve.