Stir in flour, salt and pepper, using wire whisk. Cook, stirring constantly, until mixture is smooth and bubbly. Gradually stir in broth. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese and nutmeg. Cook 1 to 2 minutes, stirring occasionally, until cheese is melted. Stir in green beans. Pour mixture into ungreased 1 1/2-quart casserole. Sprinkle with onions.