Step 1In 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.
Step 2Heat 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.
Step 3Cover 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.