Preheat over to 350

Melt 2 tablespoon 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 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.

Bake for 30 minutes or until the top is golden and cheese is bubbly.