HEAT oven to 400ºF.
COOK pasta as directed on package, omitting salt. Meanwhile, heat oil in large ovenproof skillet on medium heat. Add chicken; cook and stir 5 to 6 min. or until evenly browned. Stir in marinara sauce.
DRAIN pasta, reserving 1/2 cup cooking water. Add to ingredients in skillet; mix lightly. Stir in spinach; top with cheese mixture.
BAKE 20 min. or until heated through.