Place thawed green beans in a 13X9 inch glass pan.
Saute; onions in a large frying pan. Add flour, stir. Gradually add milk, bring to a boil. Add cheese, sour cream, sugar, and salt. Cook till thickened and cheese is melted. Pour mixture over green beans. Top with stuffing mix.
Bake at 350 degrees F (175 degrees C) for 25 to 30 minutes.