There are few things less exciting than sweet potatoes. Cauliflower, green beans, beets and Brussels sprouts fall in that category for me. I'd rather eat other vegetables. Sweet potatoes particularly turn me off — too sweet. But you can make pretty much anything taste good, especially with heavy cream, cheese, salt and pork. Adding these to sweet potatoes tempers that sweetness while still playing with it. I riff on this same recipe even more happily with Yukon Golds, adding chopped golden raisins, dried cherries, apples, or cranberries. You just need a mandoline. — Arthur Bovino
Preheat your oven to 400 degrees. Simmer 4 cups of heavy cream and stick of butter. Reduce the mixture, cooking for about 15-20 minutes. Slice the peeled potatoes on the mandoline lengthwise so they're a little less than 1/16-inch thick.
Grease the bottom and sides of a 13x9x2-inch glass baking dish baking tray with butter.
To assemble, cover the bottom of the tray with a very thin layer of the reduced cream/butter mixture. Sort sweet potato 'sheets,' and create an even layer of potatoes. Use widest potato sheets on the bottom to create a bottom layer with as much integrity as possible.
Start from the left. Or start from the right. From whatever direction you start, continue doing it, overlapping slices east-west and north-south until the first layer is done. Drizzle reduced cream/butter to cover. Evenly scatter salt and pepper. Then evenly scatter prosciutto, grated cheese and thyme. Repeat layering, starting with the potatoes, and continue until you've reached the top. Probably 2 or 3 more times, finishing with the cream-butter mixture on top and a last sprinkle of cheese.
Cover gratin with foil and bake for 30-35 minutes. Remove foil and continue to bake until the top turns golden-brown and crusty and some liquid disappears, about 20 minutes more. Let gratin stand about 5-10 minutes then serve in even squares, or use a ring mold to press down and remove shape of your choosing. Or hey, just dish it out and dig in.