HEAT oven to 400ºF.
ADD hot water to stuffing mix in large bowl; stir just until moistened.
REMOVE stems from mushrooms; chop stems. Melt butter in skillet on medium heat. Add chopped stems and garlic; cook and stir 5 min. or until tender. Add to stuffing with spinach and cheeses; mix well.
SPOON stuffing mixture into mushroom caps. Place, filled sides up, in shallow pan.
BAKE 20 min. or until mushrooms are tender and filling is heated through.