Celebrated food science writer J. Kenji Lopez-Alt brings us this gloriously golden mash up of Hasselback potatoes & a potato gratin – designed to provide you a creamy potato & singed edge in every single bite. The main revolution is placing the sliced potatoes vertically in the dish (on their edges) rather than laying them flat as you would in any other gratin; this allows the top ridges to crisp up. Some extra times is required to slice the potatoes, and a mandolin or food processor will help with this, although not strictly required. Be sure to buy extra potatoes, just in case, as you want the potatoes packed in tightly enough to stand upright.
- 3 ounces Gruyère or comté cheese (finely grated)
- 2 ounces Parmigiano-Reggiano (finely grated)
- 2 cups heavy cream
- 2 medium cloves garlic (minced)
- 1 tablespoon fresh thyme leaves (chop roughly)
- Kosher salt and black pepper
- 4 to 4 ½ pounds russet potatoes, (peel and slice 1/8 in thick if using mandolin (7-8 med, see Tip)
- 2 tablespoons butter (unsalted)
- Place oven rack in middle position, and heat to 400 degrees. In a large bowl, combine the cheeses. Place 1/3 of the cheese mixture into a separate bowl and set aside. Add the cream, thyme & garlic to the cheese mixture, and season liberally with salt & pepper. Add in the slices, and hand toss to ensure every single slice is coated with the cream mixture. Separate any slices that stick together to get the creamy mixture between them.
- Using butter, grease a 2-quart casserole dish. Taking a handful of potatoes, organize into a neat stack & place into the casserole dish with edges vertically aligned. Continue putting the potatoes in the dish, working from the perimeter towards the center, until all the potatoes have been used. Make sure the potatoes are tightly packed. If you find yourself short, slice an additional potato, coat with the cream mixture and add where needed. Next, pour any excess cream & cheese mixture evenly on top of the potatoes until it comes halfway up the sides of the casserole dish. Not all of the liquid may be required.
- Cover tightly with foil, place in the oven, and bake for 30 minutes. Next, take off the foil and allow to bake until the top turns a pale golden brown, roughly 30 minutes more. Carefully take out of oven, sprinkle with remaining cheese, and place back in oven. Bake for approx. 30 minutes longer, until crispy on top, and deep golden brown. Take out of oven and allow to rest for a few minutes before serving.
- There is natural variation in the shape of potatoes, and so how many will fit into your casserole dish will also vary. Long, thin potatoes will fill a dish more than short, round potatoes. Make sure to purchase some additional potatoes in order to fill the dish if needed. Depending on the shape and size of the potatoes you use, as well as your dish, not all of the cream mixture may be needed.