COOK and stir onions in dressing in large skillet on medium heat 5 min. or until crisp-tender. Add meat; cook on medium-high heat until browned, stirring frequently. Stir in tomato sauce and tomatoes. Bring to boil; simmer on medium-low 15 min., stirring occasionally.
MEANWHILE, cook spaghetti as directed on package, omitting salt.
REMOVE sauce from heat. Add Neufchatel; stir until melted. Drain spaghetti; place in large bowl. Top with sauce and Parmesan.