Dauphinoise or Potatoes Au Gratin is a side dish of white potatoes baked in milk or cream. This version matches the sweetness of sweet potatoes with a salty Mornay sauce enriched with lots of cheese.
Preheat the oven to 350 º F. Coat an 8 x 10-inch baking dish generously with butter.
4 tablespoons unsalted butter
Bring a large pot full of water to a boil over high heat, gently drop in the sweet potato slices, and boil for 6 minutes.
4 cups thinly sliced peeled sweet potatoes
In a saucepan over medium heat, melt the butter. Whisk in the flour until coated and cook for 2 minutes, whisking constantly. Pour in the milk and, while whisking, cook for about 4 minutes, or until the sauce thickens. Stir in the garlic, honey, mustard, nutmeg, thyme, and fine sea salt to combine.
4 tablespoons unsalted butter, ¼ cup all-purpose flour, 2 ½ cups whole milk or half-and-half, 3 garlic cloves, 2 tablespoons honey, 2 teaspoons Dijon mustard, 1 teaspoon freshly grated nutmeg, 2 teaspoons fresh thyme leaves, 1 teaspoon fine sea salt
Add the Gruyère and stir until it melts. You have just made a Mornay sauce, the same one you would make for mac and cheese.
1½ cups grated Gruyère
Layer one-third of the sweet potatoes in the bottom of the prepared baking dish and pour one-third of the sauce over the sweet potatoes. Arrange another third of the sweet potatoes in a layer over the sauce and pour another one-third of the sauce on top. Create a final layer of sweet potatoes, covering them with the remaining sauce. Sprinkle the Parmesan over the top.
½ cup grated Parmesan cheese
Cover the baking dish with a lid or aluminum foil and bake for 45 minutes to 1 hour 30 minutes, until the sweet potatoes are tender, depending on the thickness of the sweet potatoes. During the last 15 minutes or so, remove the cover while baking so the casserole browns a bit. Let rest for 15 minutes before serving.
Notes
When you buy the sweet potatoes, look for ones that are consistent in shape, meaning they don’t have fatter and thinner parts. And, if you can find the fatter ones rather than the skinny ones, you’ll get larger, rounder slices.It’s always best to grate your own cheese. It will melt better, as store bought pre shredded has an anti-caking agent which prevents the cheese melting as well as it naturally would.These can also be baked in a 13x9 casserole.Check to see if it’s done by inserting a knife into the center and it should come out clean, with no resistance.The baking time will differ depending on thickness of the slices, the shape of dish, the heat retention of baking dish, temperature of the oven, etc. That’s why there’s such a range in cooking times.Let rest for 15 minutes before serving. Be patient here. If you dig in while the dish is piping hot, you run the risk of it being soupy. Giving the dish time to cool allows the potatoes to soak up some of the sauce and the dish will firm up.Any leftovers will keep in an airtight container in the fridge for up to 5 days. Freezing is not recommended for this dish.To make ahead: Bake up the dish as directed. Allow to cool completely, cover it tightly with cling wrap. And pop in the fridge. Remove it from the fridge about 1 hour before you want to heat it. Reheat covered in foil in a 350 º F oven for 20 to 30 mins or until hot, then remove foil and bake until cheese is golden.