Preheat oven to 350 degrees. Cook and drain green beans. In a large saucepan, combine one tablespoon butter, flour, salt, pepper, sugar, onion, sour cream, and cheese. Stir and cook over low heat until ingredients are well combined and cheese is melted. Add beans, mixing well. Pour into a buttered two-quart casserole. Combine cornflakes with one tablespoon melted butter and sprinkle over bean mixture. Bake uncovered for 25 minutes.