1½ cups heavy cream
2 cloves of garlic, minced
2 pounds sweet potatoes, peeled and sliced about 1/8-inch thick
1½ teaspoons chopped fresh thyme
1½ teaspoons kosher salt
¾ teaspoon fresh ground black pepper
2 cups shredded Gruyère cheese
1. Preheat oven to 400 degrees. In a small pot bring cream and garlic just to a simmer. Remove from heat and set aside. Butter a 9x13-inch shallow casserole dish or gratin dish of similar size and set aside. Stir the cooling cream occasionally to help prevent skin from forming.
2. Build the Gratin: In the buttered casserole dish, spread one quarter of the sliced sweet potatoes out in a single, overlapping layer. Sprinkle the potatoes with 1/3 of the salt, pepper, and thyme and then about ½ cup of the grated cheese. Using another quarter of the sliced sweet potatoes, build a new overlapping layer. Again sprinkle with 1/3 of the seasonings and ½ cup of shredded cheese. Using half of the remaining sliced sweet potatoes, make another layer and sprinkle the last third of the seasonings and another ½ cup of cheese (you should have about ½ cup cheese remaining). Use the last of the sweet potatoes to make one final layer on top. Stir the garlic cream mixture then pour it evenly over the top of the gratin, being sure to cover the top layer of sweet potatoes. Shake the dish gently to distribute the cream. Sprinkle the remaining cheese over the top.
4. Cover the dish with foil and bake in the middle of the oven for 30 minutes. Remove the foil, move the gratin to the top 1/3 of the oven and bake, uncovered, for an additional 20 minutes or until the top is nicely browned and the potatoes are tender when pierced with a sharp knife. You may also place it under the broiler for a few minutes if you wish the top to be a deeper brown (but watch it closely!). Remove from oven and let cool for 30 minutes before slicing.
*Note: Make Ahead! You can slice your potatoes and completely submerge them in ice cold water, cover and refrigerate up to a couple of days. Then drain, pat dry and assemble the dish. Less to do on Thanksgiving morning, the better!