Total time: 2 hours
This golden and glorious mash-up of potato gratin and Hasselback potatoes, from the acclaimed food science writer J. Kenji López-Alt, has been engineered to give you both creamy potato and singed edge in each bite. The principal innovation here is placing the sliced potatoes in the casserole dish vertically, on their edges, rather than laying them flat as in a standard gratin, in order to get those crisp ridges on top. Allow extra time for the task of slicing the potatoes, for which it’s helpful to have a mandoline or food processor (though not necessary, strictly speaking). And do buy extra potatoes, just in case; you want to pack the potatoes tightly and keep them standing up straight.
3 oz finely grated Gruyère or comté cheese
2 oz finely grated Parmigiano-Reggiano
2 cups heavy cream
2 medium cloves garlic, minced
1 Tbsp fresh thyme leaves, roughly chopped
Kosher salt and black pepper
4 to 4 ½ pounds russet potatoes, peeled and sliced (7 to 8 medium, see note)
2 Tbsp unsalted butter
- Adjust oven rack to middle position and heat oven to 400˚F. Slice potatoes 1/8″ thick with mandoline (larger setting on mine). Combine cheeses in a large bowl. Transfer 1/3 of cheese mixture to a separate bowl and set aside. Add cream, garlic and thyme to cheese mixture. Season generously with salt and pepper. Add potato slices and toss with your hands until every slice is coated with cream mixture, making sure to separate any slices that are sticking together to get the cream mixture in between them.
- Grease a 2-quart casserole dish with butter. Pick up a handful of potatoes, organizing them into a neat stack, and lay them in the casserole dish with their edges aligned vertically. Continue placing potatoes in the dish, working around the perimeter and into the center until all the potatoes have been added. The potatoes should be very tightly packed. If necessary, slice an additional potato, coat with cream mixture, and add to casserole. Pour the excess cream/cheese mixture evenly over the potatoes until the mixture comes no more than halfway up the sides of the casserole (too much will make it boil over in oven). You may not need all the excess liquid.
- Cover dish tightly with foil and transfer to the oven. Bake for 30 minutes. Remove foil and continue baking until the top is pale golden brown, about 30 minutes longer. Carefully remove from oven, sprinkle with remaining cheese, and return to oven. Bake until deep golden brown and crisp on top, about 30 minutes longer. Remove from oven, let rest for a few minutes, and serve.
Because of variation in the shape of potatoes, the amount of potato that will fit into a single casserole dish varies. Longer, thinner potatoes will fill a dish more than shorter, rounder potatoes. When purchasing potatoes, buy a few extra in order to fill the dish if necessary. Depending on exact shape and size of potatoes and casserole dish, you may not need all of the cream mixture.