Preheat oven to 350°.
Combine green beans, onion, and water. Cover and microwave at HIGH 7 minutes.
Combine cheese and next 3 ingredients in a bowl. Stir in bean mixture. Spoon into an 8-inch square baking dish coated with cooking spray.
Combine melba rounds, onion flakes, and butter. Sprinkle crumb mixture over bean mixture. Bake at 350° for 20 to 25 minutes or until bubbly.