Coat a medium saucepan with cooking spray, and place over medium heat until hot.
Add onion and sauté 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 180°C for 25 minutes or until thoroughly heated.