COOK spaghetti in large saucepan as directed on package, omitting salt and adding peas and peppers to the boiling water for the last 2 min.
MEANWHILE, heat dressing in large skillet on medium-high heat. Add meat; stir-fry 3 min. or until meat is medium doneness. Add peanut butter and soy sauce; stir-fry 1 min. or until sauce is thickened.
DRAIN spaghetti mixture. Add to skillet with onions; toss to coat. Top with nuts.