Melt butter in large nonstick skillet on medium-high heat. Add mushrooms; cook 6 to 8 min. or until tender, stirring frequently. Remove from skillet.
Add water and zucchini to skillet; bring to boil. Add stuffing mix; stir just until moistened. Top with mushrooms, gravy and cheese; cover.
Cook on low heat 5 min. or until sauce is heated through and cheese is melted, stirring occasionally.