HEAT oven to 350ºF.
COOK pasta as directed on package, omitting salt.
MEANWHILE, melt butter in large nonstick skillet on medium heat. Add onions; cook and stir 6 min. or until crisp-tender. Blend in flour; cook and stir 1 min. Gradually stir in broth; cook 5 min. or until thickened, stirring constantly. Add 1 cup shredded cheese; cook and stir 1 min. or until melted.
DRAIN pasta. Add to ingredients in skillet along with the chicken and bacon; mix well. Transfer to 13x9-inch baking dish sprayed with cooking spray; top with remaining shredded cheese. Cover.
BAKE 40 min., uncovering and topping with Parmesan for the last 10 min. Top with tomatoes and cilantro.