I grew up having au gratin potatoes that were delicious, lots of cheese, definitely addictive. I developed this recipe because I love sweet potatoes and couldn’t ever find an au gratin recipe that incorporated them in a simple and purist way. This recipe has very few ingredients, the sweet potato is unexpected and still the star. If you can find Japanese sweet potatoes, they really add a special something to this very simple dish. Their flesh is white and delicate so they just meld perfectly with the waxy yukon golds. Often I layer in a tbsp of chopped fresh thyme for freshness.
2 lg yukon gold potatoes
1 1/2 japanese sweet potatoes (the typical orange type works fine too)
1 1/2 cups heavy cream
3 tbsp butter
1/4 cup grated Parmesan
plenty of salt and freshly ground pepper
garlic clove smashed
1 tbsp thyme leaves (chopped and divided)
Peel your sweet potatoes. No need to peel the Yukons. This gratin is most beautiful when baked in a round ceramic dish, but as long as your baking dish or casserole is somewhat shallow, you are good. Smash your garlic and rub the inside of your dish. Soften 1 TBSP of butter and rub that all over the inside of the baking dish as well. Cut your potatoes as thin as possible, perhaps with a mandolin. I find the thinner they are the more they melt in your mouth. Layer the potatoes by going around the outer edge first and overlapping each piece of potato. Either alternating sweet and yukons each row or alternating them each piece. Add salt, pepper and some parmesan in between layers + 1/2 tbsp of chopped thyme (optional). Once all the potatoes are in, pour the heavy cream over the potatoes add another big pinch of salt and pepper and the remaining parmesan. Dob the remaining butter over the top. Bake for 1 hour.