Smaller batch - see Potato Gratin Galway Blazers for 5lb version
Preheat oven to 375F
Place cream, milk, leek, salt, pepper, potato, thyme, bay leaf, and garlic in Dutch oven and bring to a boil
Cover, reduce heat and simmer 10 min or until potatoes are tender
Discard thyme and bay leaf.
Spoon half mixture into a 13×9 baking dish coated with cooking spray
Sprinkle with half of the cheese.
Top with remaining potatoes and remaining cheese.
Bake for 1 hour or until golden brown.
Stand for 10 minutes before serving