Grandma's Green Bean Casserole

Green Beans:

• 2 tablespoons butter
• 2 tablespoons all-purpose flour
• 1 cup sour cream
• ¼ cup diced onion
• 1 teaspoon salt
• 1 teaspoon white sugar
• 3 (14.5 ounce) cans French-style green beans, drained

Topping:

• 2 cups shredded Cheddar cheese
• ½ cup crumbled buttery round crackers
• 1 tablespoon butter, melted