Heat oven to 350°F. In 12-inch skillet, heat butter over medium heat. Add leeks; cook 5 to 7 minutes, stirring occasionally, until leeks are tender and soft.
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.
Bake 20 to 25 minutes or until bubbly.