In a large salad bowl, combine spinach, leaf lettuce, sliced red onion, and pepper strips. Cover and chill salad up to 2 hours.
Cut chicken into bite-sized strips. Toss chicken with rosemary and lemon-pepper seasoning. In a wok or 10-inch skillet stir-fry chicken strips and garlic in hot oil over medium-high heat for 2 to 3 minutes or until chicken is tender and no longer pink. Remove chicken from skillet. Add to salad mixture.
For dressing, add vinegar and water to skillet, stirring to scrape up any browned bits. Pour dressing over salad. Toss gently to mix. Transfer to individual salad plates. Garnish with fresh rosemary, if desired. Makes 4 servings.