Grandma's Green Bean Casserole

INGREDIENTS

• 2 tablespoons butter
• 2 tablespoons all-purpose flour
• 1 teaspoon salt
• 1 teaspoon white sugar
• 1/4 cup onion, diced
• 1 cup sour cream
• 3 (14.5 ounce) cans French style green beans, drained
• 2 cups shredded Cheddar cheese
• 1/2 cup crumbled buttery round crackers
• 1 tablespoon butter, melted