In medium bowl, mix chicken and 1/4 cup of the dressing; toss to coat. Let stand at room temperature 10 minutes to marinate.
In 8-inch nonstick skillet, heat oil over medium-high heat until hot. Remove chicken from marinade with slotted spoon and add to skillet; discard remaining marinade. Cook and stir chicken 4 to 6 minutes or until browned and no longer pink in center. Remove from heat. Add remaining 1/2 cup dressing; stir to mix.
Divide cabbage and spinach among 4 serving plates. Top with chicken mixture. Sprinkle with onions and peanuts. If desired, serve with additional dressing.