COOK pasta in large saucepan as directed on package, omitting salt and adding spinach to the boiling water for the last minute.
MEANWHILE, brown sausage with onions in large nonstick skillet on medium heat. Drain; return sausage mixture to skillet. Stir in spaghetti sauce; cook 10 min. or until heated through, stirring occasionally.
DRAIN pasta mixture. Serve topped with sauce and cheeses.