Pre-heat the oven to 350°F.
In a large skillet, over medium heat, melt the 2 tbsp. butter.
Stir in flour until smooth; cook for one minute.
Stir in the salt, sugar, onion and sour cream.
Add green beans; stir to coat.
Transfer the mixture to a 2-½-quart casserole dish.
Spread shredded cheese over the top.
In a small bowl, toss together cracker crumbs and remaining butter; sprinkle over the cheese.
Bake in the pre-heated oven until topping is golden and cheese is bubbly; about 30 minutes.