Heat oven to 350.
Melt 2 tbsp butter over medium heat. Saute onion until transparent, then stir in flour until smooth and cook one minute.
Add in salt, sugar, and sour cream. Stir in green beans and mix thoroughly.
Transfer mixture to casserole dish. Sprinkle with cheddar cheese.
Bake 30 minutes.
Sprinkle French's onions on top, bake an additional 3-5 minutes.