Bring a pot of cold water to a boil with the diced potatoes. Cook until very tender.
Preheat oven to 425°F.
While the potatoes are cooking prepare the leeks by trimming off the excess green parts from the top. Remove any excess tough or outer parts that might not look so fresh around the bulbs. Trim the root end off the bottom of the bulbs and then slice in half lengthwise and wash off dirt and debris from between the layers of the bulb.
Pat dry and coat your baking dish and the leek halves with 1 tablespoon of vegan butter.
Drain potatoes from the boiling water and add to a high-powered blender with garlic cloves, remaining 3 tablespoons of vegan butter, lemon juice, non-dairy milk, vegetable stock, nutritional yeast, cornstarch, spices, and salt and pepper. Blend on high until very smooth.
Pour ⅓ of the sauce in the bottom of the baking dish. Arrange the leeks in the baking dish. Then pour the remaining sauce over top and even it out. The leeks will be fully coated in sauce.
Combine the topping ingredients together except the dill, and sprinkle over the top in an even layer.
If your dish is quite full to the top edge place a baking sheet on the bottom rack of the oven and place the gratin above on the middle rack. This will catch any bubbling sauce that might over flow. Bake for 30 minutes or until the top is golden brown and the sauce is bubbling.
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Notes
Depending on your oven you might need to move the dish to the top rack for another 5 to 10 minutes to get the top more crispy and brown.