COOK pasta as directed on package, omitting salt.
MEANWHILE, heat 1/4 cup dressing in large skillet on medium heat. Add onions; cook 5 min. or until golden brown, stirring occasionally. Remove from skillet. Add remaining dressing and tomatoes to skillet; cook and stir 3 min. or until heated through.
DRAIN pasta, reserving 1/2 cup cooking water. Toss pasta with vegetables and basil in large bowl. Gradually stir in reserved cooking water, if needed until pasta mixture is of desired consistency. Top with cheese.