PLACE potatoes in large saucepan. Add enough water to completely cover potatoes. Bring to boil on high heat. Reduce heat to medium-low; cook 15 min. or until potatoes are tender.

DRAIN potatoes; return to saucepan. Add sour cream, cream cheese and milk. Mash with potato masher to desired consistency. Spoon into microwaveable serving bowl; cover with plastic wrap. Refrigerate until ready to use.

MEANWHILE, melt butter in large skillet on medium-high heat; add mushrooms. Cook 5 min. or until mushrooms are tender, stirring frequently. Spoon into small bowl; cover. Refrigerate until ready to use.

VENT plastic wrap covering potatoes. Microwave on HIGH 6 to 8 min. or until heated through; stir. Top with mushrooms. Microwave, uncovered, an additional 2 to 3 min. or until heated through. Sprinkle with thyme.