3/4 cup freshly grated Parmigiano-Reggiano cheese (2 1/2 ounces)
Extra-virgin olive oil, for brushing
2 pounds Yukon Gold potatoes, peeled and sliced 1/8 inch thick
1 head of cauliflower (2 1/2 pounds)—halved lengthwise, cored and sliced 1/8 inch thick, crumbled pieces reserved
1 large white onion, very thinly sliced
How to Make It
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. Generously season the cream with salt and white pepper.
Preheat the oven to 350°. In a bowl, mix together all three cheeses. Lightly brush a 7-quart, enameled cast-iron casserole with olive oil. Arrange one slightly overlapping layer of potatoes in it followed by one layer of the cauliflower (including the crumbles) . Scatter some of the onion on top and sprinkle with 3/4 cup of the cheese. Drizzle with 1/2 cup of the vadouvan cream. Repeat this layering 3 more times with the remaining potatoes, cauliflower, onion, cheese and cream. Pour the remaining cream mixture all over the top.
Cover with foil and poke a few holes in it. Bake the gratin for 1 1/2 hours, until tender. Uncover and cook for 30 minutes longer, until the cream has been absorbed and the gratin is golden brown on top. Transfer to a rack and let stand for 20 minutes before serving.