Mix first 6 ingredients in small bowl for sauce. Mix egg white, cold water, and cornstarch in medium bowl; mix in chicken. Let sauce and chicken stand 15 minutes.
Heat wok or large nonstick skillet over medium-high heat 1 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 1 minute.