Mix first 6 ingredients in a small bowl for sauce; whisk cold water and cornstarch together and add to sauce; whisk together.
Cut chicken into strips, sprinkle with pepper.
Heat wok over medium high heat one minute. Add oil and tilt to coat. Add chicken, stir fry 2 minutes. Add mushrooms, bell pepper and garlic, stir fry until mushrooms are browned at edges, about 3 minutes. Add sauce, cook until sauce is reduced to glaze, about 1 1/2 minutes. Mix in spinach and toss one minute.