1. Heat a large skillet over high heat. Add 1 tablespoon oil to pan; swirl to coat. Add chicken; sprinkle with 1/4 teaspoon pepper and 1/8 teaspoon salt. Cook 3 minutes or until chicken is just done, stirring occasionally. Remove chicken from pan.
2. Add mushrooms, 1/4 teaspoon pepper, and 1/8 teaspoon salt to pan; stir-fry 6 minutes or until mushrooms brown and most of liquid evaporates. Add 1 tablespoon oil, ginger, and garlic; cook 30 seconds, stirring constantly. Return chicken to pan; add soy sauce, mirin, and vinegar. Cook 2 minutes, scraping pan to loosen browned bits. Remove pan from heat; stir in spinach and onions.