HEAT oven to 350ºF.
COOK noodles in large saucepan as directed on package, omitting salt and adding peas to the boiling water the last minute. Meanwhile, microwave cream cheese, milk and onion powder in microwaveable bowl on HIGH 1-1/2 to 2 min. or until cream cheese is melted and mixture is well blended, stirring after 1 min.
DRAIN noodle mixture; place in 3-qt. casserole sprayed with cooking spray. Add cream cheese sauce, chicken, mushrooms and 1 cup cheddar; mix well. Top with remaining cheddar. Spread bread with butter; sprinkle with Parmesan. Cut diagonally into quarters; arrange on top of casserole.
BAKE 25 to 30 min. or until bread is golden brown and casserole is heated through.