HEAT oven to 350°F.
COOK spaghetti in large saucepan as directed on package. Meanwhile, cook chicken and vegetables in large nonstick skillet 5 min. or until chicken is no longer pink. Remove from skillet; cover to keep warm.
BRING Neufchatel, flour and broth to boil in same skillet, stirring constantly with whisk; simmer on medium-low heat 5 min., stirring frequently.
DRAIN spaghetti; return to saucepan. Add Neufchatel sauce, chicken mixture and 2 Tbsp. Parmesan; mix well. Spoon into 2-qt. casserole; cover.
BAKE 25 min. Top with mozzarella and remaining Parmesan; bake, uncovered, 2 min. or until mozzarella is melted