Green Bean Casserole

I found this recipe on AllRecipes about a decade ago and have been making it for every holiday ever since.  It's really good.

Ingredients
2 tbs butter
2 tbs all-purpose flour
1 tsp salt
1 tsp white sugar
1/4 cup onion, diced
1 cup sour cream (low-fat works fine)
3 (14.5 oz) cans French-style green beans, drained
2 cups shredded cheddar cheese
1/2 cup crumbled buttery round crackers
2 tbs butter, melted

Directions

1. Preheat oven to 350 degrees F (175 degrees C).
2. Melt 2 tablespoons butter in a large skillet over medium heat. Stir in flour until smooth, and cook for one minute. Add onion. Combine the salt, sugar, and sour cream, and add to skillet. Add green beans, and stir to coat.
3. Transfer the mixture to a 2 1/2 quart casserole dish. Spread shredded cheese over the top. In a small bowl, toss together cracker crumbs and remaining butter, and sprinkle over the cheese.
4. Bake for 30 minutes in the preheated oven, or until the top is golden and cheese is bubbly.