This dish is a riff on pomme dauphinois, a labor-intensive southern French classic of paper-thin sliced potatoes layered with Gruyère cheese, cream and garlic. To that base, add Parmesan and Cantal cheese if you can find it and cauliflower, and then spike it with curry leaves and vadouvan, a kind of French curry powder-ish spice blend that you should definitely find for your cupboard.
In a medium saucepan, combine the cream with the vadouvan, curry leaves and garlic and bring to a simmer. Let stand off the heat for 15 minutes to infuse. Season the cream with 1 teaspoon salt and ½ teaspoon white pepper, or to taste.
Heat the oven to 350 degrees. In a bowl, mix together all three cheeses. Lightly brush a 5½-quart enameled cast-iron casserole with olive oil. Arrange one slightly overlapping layer of potatoes in the casserole dish, followed by a layer of the cauliflower (including the crumbles). Scatter some of the onion on top and sprinkle with ¾ cup of the cheese. Drizzle with ½ cup of the vadouvan cream. Repeat layering 3 more times with the remaining potatoes, cauliflower, onion, cheese and cream. Pour the remaining cream mixture over the top.
Cover with foil and poke a few holes in it. Bake the gratin until tender, about 1½ hours. Uncover and cook until the cream has been absorbed and the gratin is golden brown on top, 30 to 60 minutes longer. Transfer to a rack and let stand for 20 minutes before serving.
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