Coat a medium saucepan with cooking spray, and place over medium heat until hot. Add onion; saute 5 minutes or until tender. Add flour, and cook 1 minute, stirring constantly. Gradually add the milk, stirring until blended. Stir in cheese, sour cream, sugar, and salt; cook 5 minutes or until thickened and bubbly, stirring constantly.
Place green beans in an 8-inch square baking dish; pour the sauce over beans. Combine stuffing mix, margarine, and egg white in a bowl; stir well, and sprinkle over green bean mixture. Bake at 350° for 25 minutes or until thoroughly heated.