Preheat oven to 400 degrees F. Mix stuffing mix and hot water in large bowl; set aside. Remove stems from mushrooms; chop stems. Melt butter in large skillet on medium heat. Add chopped stems and the garlic; cook and stir until tender. Add to prepared stuffing along with the spinach and cheeses; mix well.
Spoon evenly into mushroom caps. Place, filled-sides up, in shallow baking pan.
Bake 20 min. or until mushrooms are tender and filling is heated through.