Hasselback Potato Gratin

Ingredients

• 3 ounces finely grated Gruyère or Comté cheese
• 2 ounces finely grated Parmigiano-Reggiano cheese
• 2 cups heavy cream
• 2 medium cloves garlic, minced
• 1 tablespoon fresh thyme leaves, roughly chopped
• Kosher salt and freshly ground black pepper
• 3 to 3 1/2 pounds russet potatoes, peeled and sliced 1/8 inch thick on a mandoline slicer
• 2 tablespoons unsalted butter

Directions

1. Set an oven rack in the middle and heat the oven to 400 degrees F. Toss the cheeses together in a large bowl. Scoop 1/3 of the cheese mixture into a separate bowl and set it aside. Stir the cream, garlic, and thyme into the cheese left in the large bowl. Be generous with the salt and pepper. Add the potato slices, then use your hands to turn them until the cream mixture coats every slice. Pull apart any pieces that have clung to each other so the cream can get between them.
2. Butter a 2-quart casserole dish. Gather a handful of potatoes, straighten them into a tidy stack, and nestle the stack into the casserole so the cut edges stand upright. Keep adding potatoes, moving from the outside edge toward the middle, until all the potatoes are in the dish. The potatoes need to be packed very tightly. If you need more to fill gaps, slice additional potato, coat it with cream mixture, and tuck it into the casserole. Drizzle the extra cream/cheese mixture over the potatoes in an even layer, stopping when it reaches half way up the casserole's sides. Do not worry if some of the liquid is left behind.
3. Seal the dish well with foil and put it in the oven. Bake for 30 minutes. Take off the foil, then bake again until the surface turns a light golden brown, about 30 minutes longer. Pull the dish out carefully, scatter the reserved cheese over the top, and slide it back into the oven. Bake until the top is crisp and a deeper golden brown, about 30 minutes longer. Take it out of the oven, give it a few minutes to rest, and serve.