MELT butter in large skillet on medium heat. Add mushrooms, onions and celery; cook 6 to 8 min. or until onions and celery are crisp-tender, stirring occasionally.
RESERVE 1 cup vegetable mixture. Add next 5 ingredients to vegetable mixture remaining in skillet; stir. Bring to boil; simmer on medium-low heat 15 min. or until potatoes are tender.
ADD 1/3 of the potato mixture to blender; blend until smooth. Pour into large saucepan. Repeat with remaining potato mixture.
WHISK in Neufchatel; cook and stir 2 min. or until melted. Stir in reserved vegetable mixture and parsley.