Preheat oven to 375°.
Place potatoes in a Dutch oven, and cover with water. Bring to a boil; remove from heat. Drain; set aside.
Heat pan over medium heat. Coat pan with cooking spray. Add leek and oil, stirring to coat leek. Cover and cook 10 minutes or until the leek is tender. Remove from pan; set aside.
Add broth, mustard, pepper, and nutmeg to pan, stirring with a whisk until well blended. Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a medium bowl; gradually add milk, stirring with a whisk to form a slurry. Add slurry to broth mixture, stirring to combine; bring to a boil. Reduce heat, and simmer 3 minutes or until mixture thickens, stirring constantly.
Pour 1 cup milk mixture into bottom of a 13 x 9-inch baking dish coated with cooking spray; arrange 2 1/3 cups potatoes over milk mixture. Top potatoes with 1 1/2 cups ham and 1 cup leek; sprinkle with 1/2 cup cheese. Repeat layers once, ending with cheese, and top with remaining potatoes. Pour the remaining milk mixture over potatoes; sprinkle with remaining cheese. Bake at 375° for 45 minutes or until bubbly and golden brown. Let stand 15 minutes before serving.