Prep tips

• Peel your potatoes first, and let them soak whole in a large bowl of cold water until you are ready to slice. Potatoes oxidize and turn a rusty brown not too long after they’ve been exposed to air. Soaking them in water helps prevent this, and buys you time to get everything else together. I recommend prepping everything else first, and waiting to the very last minute to slice the potatoes so that they’re not exposed to air for too long.
• To make things easier, I highly recommend you use a mandoline to help you slice your potatoes evenly and accurately. Keep your slices between 1/8 – 1/16 of an inch for best results.

1. Preheat oven to 400 F and adjust the oven rack to the middle position. Prepare a 3-quart gratin or baking dish by brushing the interior with about 2 tablespoons of olive oil. 
2. In a medium bowl, toss the grated gruyere, white cheddar, and parmigiana reggiano with the cornstarch until evenly coated. Set aside.
3. In a large bowl, add the sliced potatoes. Pour in the heavy cream and half and half. Add the fresh dill, parsley, rosemary, thyme, shallots, garlic, nutmeg, kosher salt, pepper, and cheese/cornstarch mixture. Toss everything together, separating the potatoes as necessary to make sure they’re all coated. Everything should be evenly dispersed. 
4. Gather small handfuls of the potatoes in shingled stacks, then place into the greased baking dish, making sure they’re tightly packed and their edges are vertical like a hasselback potato. It’s easier to start placing them around the outer edge of the dish, and working towards the center. Repeat until all of the potatoes are used, and make sure they’re all tightly packed so they can stand themselves up. Pour the excess herb, cheese and cream mixture evenly over the surface of the potatoes until it comes halfway up the sides of the baking dish. There might be some leftover liquid — if so, discard it, but feel free to scrape out any cheese leftover.
5. Cover the dish tightly with foil and place in the preheated oven on the middle rack. Bake for 40 minutes, then remove foil and continue baking an additional 35 minutes or until the tips of the potatoes are starting to turn golden brown. Remove the dish from the oven, and sprinkle with the cheese topping (4 ounces cheddar, 2 tablespoons pecorino). Place dish back in the oven, uncovered, and cook an additional 15-20 minutes, or until the cheese is spotty brown and the entire mixture is bubbly and the potatoes are tender.
6. Remove the potatoes from the oven, and allow them to cool for 15-20 minutes before serving.