Microwave potato slices and 1 cup water in a shallow glass dish covered with plastic wrap at HIGH 10 minutes or until tender, stirring once after 5 minutes. Drain and set aside.
Melt 2 tablespoons butter in a large saucepan over medium-high heat; add onion, and sauté 5 minutes or until tender. Add potato slices, and toss gently; remove mixture from saucepan.
Melt remaining 2 tablespoons butter in saucepan over medium heat. Gradually whisk in milk and gravy mix; whisk until smooth. Add shredded cheese, whisking until cheese is melted. Remove from heat, and stir in potato mixture. Spoon into a lightly greased 2 1/2-quart or 9-inch square baking dish. Combine breadcrumbs and dried parsley flakes; sprinkle over potato mixture.
Bake at 400° for 25 to 30 minutes or until bubbly.