COOK pasta as directed on package, omitting salt.
MEANWHILE, melt butter in medium saucepan on medium heat. Add onions; cook and stir 7 min. or until crisp-tender. Add cream cheese and milk; cook 5 min. or until mixture comes to boil and cream cheese is completely melted, stirring frequently. Add Parmesan; cook 3 min., stirring frequently. Add ham, peas, pepper and nutmeg; cook 3 min. or until heated through, stirring frequently.
DRAIN pasta; place in large bowl. Add cream cheese sauce; toss to coat.