Cook and drain pasta as directed on package.
Meanwhile, in 12-inch nonstick skillet, heat oil over medium-high heat. Cook onion in oil about 2 minutes, stirring frequently. Add dressing, tomatoes and chicken. Cook 2 to 3 minutes, stirring occasionally, until hot. Stir in spinach; cook 1 to 2 minutes or until spinach starts to wilt.
In large serving bowl, toss pasta with chicken mixture. Top with cheese.