Melt 2 tbsp butter in a large skillet over medium-high heat. Stir in flour until smooth, and cook for 1 minute. Stir in the salt, sugar, onion, & sour cream. Add green beans & stir to coat.
Transfer the mixture to a casserole dish. Spread shredded cheese over the top. In a small bowl, toss together cracker crumbs & remaining butter, & sprinkle over the cheese.
Bake for 30 minutes or until the top is golden & cheese is bubbly.