Preheat oven to 375°.
Place milk, leek, salt, pepper, potato, thyme, bay leaf, and garlic in a Dutch oven; bring to a boil. Cover, reduce heat, and simmer 10 minutes or until potatoes are tender. Discard thyme and bay leaf.
Spoon half of the potato mixture into a 13 x 9-inch baking dish coated with cooking spray. Sprinkle with 1/2 cup cheese. Top with remaining potato mixture and remaining 1/2 cup cheese. Bake at 375° for 1 hour or until golden brown. Let stand for 10 minutes before serving.