Pre-Heat oven to 400 degrees. Mix stuffing mix and water in large bowl; set aside. Remove stems from mushrooms; chop stems fairly fine. Melt butter in skillet on medium heat. Add chopped stems and garlic; then cook and stir until tender. Add this to prepared stuffing with spinach and cheeses; mix well.
Spoon into mushroom caps. Place, filled-sides up, in shallow pan. Bake 20 minutes or until mushrooms are tender and filling is heated through.
I usually serve this with a thinned out homemade Ranch dressing in a dish with a spoon so it make be drizzled over the caps if guest desire.