Preheat the oven to 180°C. Heat the oil in a large frypan and sauté the sliced leeks and garlic. Season with the celery salt or all-purpose seasoning and cook the leeks and garlic for 5-10 minutes over a gentle heat until softened and fragrant.
Grease a medium ovenproof dish with the butter or spray oil. Arrange a layer of the potato slices in the base of the dish, overlapping the slices.
Divide the leek mixture in half and spread one half over the layer of potatoes. Place another layer of potatoes on top and spread over the remaining leek mixture. Top with a third layer of the potatoes.
Whisk together the milk and cream and slowly pour the mixture over the leek and potato bake. Allow the liquid to soak through the layers.
Cover the dish with foil and bake in the oven for 30 minutes. Remove the foil and sprinkle over the panko crumbs and cheese, return the bake to the oven for 20-30 minutes until the topping is golden and the potato slices are tender when pierced.