Combine broth and sliced leeks in a medium saucepan; bring to a boil. Cover, reduce heat, and simmer 5 minutes. Combine milk and next 5 ingredients, stirring with a whisk until smooth. Add to broth mixture, stirring well. Cook 3 minutes or until mixture is thickened and bubbly, stirring constantly. Remove from heat.
Layer half of potato in a 1 1/2-quart oval au gratin or baking dish coated with cooking spray; pour half of leek mixture over potato. Repeat layers with remaining potato and leek mixture. Cover and bake at 350° for 55 minutes. Uncover and bake 15 minutes or until potato is tender and lightly browned.