Preheat oven to 350 degrees F (175 degrees C)
Melt 2 tablespoons butter in a large skillet over medium heat. Stir in flour until smooth, and cook for one minute. Stir in the salt, sugar, onion, and sour cream. Add green beans, and stir to coat.
Transfer the mixture to an 8 x8 glass baking dish . Spread shredded cheese over the top. Top with french fried onions and sprinkle over the cheese.
Bake for 30 minutes in the preheated oven, or until the top is golden and cheese is bubbly