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.
Kraft kitchens tips:
SUBSTITUTE Prepare using STOVE TOP Savory Herbs Stuffing Mix.
LEFTOVER STUFFING? Mushrooms vary in size. Any leftover stuffing mixture can be served as a side dish with baked chicken.