HEAT oven to 400°F. Mix stuffing mix and water; set aside. Remove stems from mushrooms; chop stems. Melt butter in skillet on medium heat. Add chopped stems and garlic; cook and stir until tender. Add to prepared stuffing along with 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.