Melt butter in large saucepan on medium heat. Add mushrooms and onion; cook and stir 5 minutes or until tender.
Mix Gravy Mix, water and turkey drippings. Add to saucepan; mix well. Stirring frequently, cook on medium-high heat until gravy comes to boil. Reduce heat to low; simmer 5 minutes or until thickened, stirring occasionally. (Gravy will continue to thicken upon standing.)